Semiconductor device or display device including the same

ABSTRACT

A method for manufacturing a novel semiconductor device is provided. The method includes a first step of forming a first oxide semiconductor film over a substrate, a second step of heating the first oxide semiconductor film, and a third step of forming a second oxide semiconductor film over the first oxide semiconductor film. The first to third steps are performed in an atmosphere in which water vapor partial pressure is lower than water vapor partial pressure in atmospheric air, and the first step, the second step, and the third step are successively performed in this order.

BACKGROUND OF THE INVENTION 1. Field of the Invention

One embodiment of the present invention relates to a semiconductordevice including an oxide semiconductor film. Another embodiment of thepresent invention relates to a display device including thesemiconductor device.

Note that one embodiment of the present invention is not limited to theabove technical field. The technical field of one embodiment of theinvention disclosed in this specification and the like relates to anobject, a method, or a manufacturing method. In addition, one embodimentof the present invention relates to a process, a machine, manufacture,or a composition of matter. In particular, one embodiment of the presentinvention relates to a semiconductor device, a display device, alight-emitting device, a power storage device, a memory device, adriving method thereof, or a manufacturing method thereof.

In this specification and the like, a semiconductor device generallymeans a device that can function by utilizing semiconductorcharacteristics. A semiconductor element such as a transistor, asemiconductor circuit, an arithmetic device, and a memory device areeach an embodiment of a semiconductor device. An imaging device, adisplay device, a liquid crystal display device, a light-emittingdevice, an electro-optical device, a power generation device (includinga thin film solar cell, an organic thin film solar cell, and the like),and an electronic device may each include a semiconductor device.

2. Description of the Related Art

As a semiconductor material that can be used in a transistor, an oxidesemiconductor has been attracting attention. For example, PatentDocument 1 discloses a semiconductor device whose field-effect mobility(in some cases, simply referred to as mobility or μFE) is improved bystacking a plurality of oxide semiconductor layers, among which theoxide semiconductor layer serving as a channel contains indium andgallium where the proportion of indium is higher than the proportion ofgallium.

Non-Patent Document 1 discloses a solid solution range in theIn₂O₃—Ga₂ZnO₄—ZnO system.

Non-Patent Document 2 discusses a structure in which an active layer ofa transistor includes two layers of oxide semiconductors of indium zincoxide and IGZO.

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2014-007399

Non-Patent Document

-   [Non-Patent Document 1]M. Nakamura, N. Kimizuka, and T. Mohri, “The    Phase Relations in the In₂O₃—Ga₂ZnO₄—ZnO System at 1350° C.,” J.    Solid State Chem., 1991, Vol. 93, pp. 298-315.-   [Non-Patent Document 2] John F. Wager, “Oxide TFTs: A Progress    Report”, Information Display 1/16, SID 2016, January/February 2016,    Vol. 32, No. 1, pp. 16-21

SUMMARY OF THE INVENTION

In Non-Patent Document 2, a channel-protective bottom-gate transistorachieves high field-effect mobility (μ=62 cm²V⁻¹s⁻¹). An active layer ofthe transistor is a two-layer stack of an indium zinc oxide and an IGZO,and the thickness of the indium zinc oxide where a channel is formed is10 nm. However, the S value (the subthreshold swing (SS)), which is oneof transistor characteristics, is as large as 0.41 V/decade. Moreover,the threshold voltage (Vth), which is also one of transistorcharacteristics, is −2.9 V, which means that the transistor has anormally-on characteristic.

The field-effect mobility of a transistor that uses an oxidesemiconductor film as a channel region is preferably as high aspossible. However, when the field-effect mobility is increased, thetransistor has a problem with its characteristics, that is, thetransistor tends to be normally on. Note that “normally on” means astate where a channel exists without application of a voltage to a gateelectrode and a current flows through the transistor.

Furthermore, in a transistor that uses an oxide semiconductor film in achannel region, oxygen vacancies formed in the oxide semiconductor filmadversely affect the transistor characteristics. For example, oxygenvacancies formed in the oxide semiconductor film are bonded to hydrogento serve as a carrier supply source. The carrier supply source generatedin the oxide semiconductor film causes a change in the electricalcharacteristics, typically, shift in the threshold voltage, of thetransistor including the oxide semiconductor film.

Too many oxygen vacancies in an oxide semiconductor film, for example,shift the threshold voltage of the transistor in the negative direction,causing normally-on characteristics. Thus, it is preferable that achannel region in an oxide semiconductor film especially include fewoxygen vacancies or include oxygen vacancies such that normally-oncharacteristics are not caused.

In view of the foregoing problems, an object of one embodiment of thepresent invention is to improve field-effect mobility and reliability ina transistor including an oxide semiconductor film. Another object ofone embodiment of the present invention is to prevent a change inelectrical characteristics of a transistor including an oxidesemiconductor film and to improve reliability of the transistor. Anotherobject of one embodiment of the present invention is to provide asemiconductor device with low power consumption. Another object of oneembodiment of the present invention is to provide a novel semiconductordevice. Another object of one embodiment of the present invention is toprovide a novel method for manufacturing a semiconductor device. Anotherobject of one embodiment of the present invention is to provide a methodfor manufacturing a highly reliable semiconductor device at relativelylow temperature.

Note that the description of the above object does not disturb theexistence of other objects. One embodiment of the present invention doesnot necessarily achieve all of these objects. Objects other than theabove objects will be apparent from and can be derived from thedescription of the specification and the like.

One embodiment of the present invention is a manufacturing method of asemiconductor device including a first step of forming a first oxidesemiconductor film over a substrate, a second step of heating the firstoxide semiconductor film, and a third step of forming a second oxidesemiconductor film over the first oxide semiconductor film. The first tothird steps are performed in an atmosphere in which water vapor partialpressure is lower than water vapor partial pressure in atmospheric air,and the first step, the second step, and the third step are successivelyperformed in this order.

In the above embodiment, the second step is performed in a temperaturerange of higher than or equal to 100° C. and lower than or equal to 450°C. for a treatment time of longer than or equal to 1 minute and shorterthan or equal to 60 minutes.

Another embodiment of the present invention is a manufacturing method ofa semiconductor device including a first step of forming a first oxidesemiconductor film over a substrate, a second step of heating the firstoxide semiconductor film, a third step of forming a second oxidesemiconductor film over the first oxide semiconductor film, and a fourthstep of heating the substrate. The first to fourth steps are performedin an atmosphere in which water vapor partial pressure is lower thanwater vapor partial pressure in atmospheric air, and the fourth step,the first step, the second step, and the third step are successivelyperformed in this order.

In the above embodiment, it is preferable that the second step and thefourth step are each independently performed within a temperature rangeof higher than or equal to 100° C. and lower than or equal to 450° C.for a treatment time of longer than or equal to 1 minute and shorterthan or equal to 60 minutes.

In any of the above embodiments, it is preferable that each of the firstoxide semiconductor film and the second oxide semiconductor film beformed by a sputtering method.

In any of the above embodiments, it is preferable that the first oxidesemiconductor film be formed under a lower oxygen partial pressure thanthe second oxide semiconductor film. In any of the above embodiments, itis preferable that the first oxide semiconductor film be formed with anoxygen flow rate ratio higher than or equal to 0% and lower than orequal to 30% and the second oxide semiconductor film be formed with anoxygen flow rate ratio higher than 30% and lower than or equal to 100%.

In any of the above embodiments, it is preferable that at least one ofthe first oxide semiconductor film and the second oxide film be formedat a temperature lower than 100° C. In any of the above embodiments, itis preferable that at least one of the first oxide semiconductor filmand the second oxide semiconductor film be formed without performingheating intentionally.

In any of the above embodiments, it is preferable that the first oxidesemiconductor film be formed such that the first oxide semiconductorfilm has lower crystallinity than the second oxide semiconductor film.In any of the above embodiments, it is preferable that the first oxidesemiconductor film be formed such that the first oxide semiconductorfilm includes a nanocrystal and the second oxide semiconductor film beformed such that the second oxide semiconductor film includes ac-axis-aligned crystal.

In any of the above embodiments, it is preferable that at least one ofthe first oxide semiconductor film and the second film be formed with anIn-M-Zn oxide (the M is Ga, Al, Y, or Sn) target.

In the above embodiment, it is preferable that an atomic ratio of In toM and Zn be 4:2:4.1 or a neighborhood of 4:2:4.1. In the aboveembodiment, it is preferable that an atomic ratio of In to M and Zn be5:1:7 or a neighborhood of 5:1:7

One embodiment of the present invention can improve field-effectmobility and reliability in a transistor including an oxidesemiconductor film. One embodiment of the present invention can preventa change in electrical characteristics of a transistor including anoxide semiconductor film and improve the reliability of the transistor.One embodiment of the present invention can provide a semiconductordevice with low power consumption. One embodiment of the presentinvention can provide a novel semiconductor device. One embodiment ofthe present invention can provide a method for manufacturing a novelsemiconductor device. One embodiment of the present invention canprovide a method for manufacturing a highly reliable semiconductordevice at relatively low temperature.

Note that the description of these effects does not preclude theexistence of other effects. One embodiment of the present invention doesnot necessarily have all of these effects. Other effects will beapparent from and can be derived from the description of thespecification, the drawings, the claims, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart illustrating a method for manufacturing asemiconductor device.

FIG. 2 is a flowchart illustrating a method for manufacturing asemiconductor device.

FIG. 3 is a top view of a deposition apparatus.

FIGS. 4A to 4C are cross-sectional views of a deposition apparatus.

FIGS. 5A to 5C are a top view and cross-sectional views illustrating asemiconductor device.

FIGS. 6A to 6C are a top view and cross-sectional views illustrating asemiconductor device.

FIGS. 7A to 7C are a top view and cross-sectional views of asemiconductor device.

FIGS. 8A to 8C are a top view and cross-sectional views illustrating asemiconductor device.

FIGS. 9A to 9C are a top view and cross-sectional views illustrating asemiconductor device.

FIGS. 10A to 10C are a top view and cross-sectional views illustrating asemiconductor device.

FIGS. 11A to 11C are cross-sectional views illustrating a method formanufacturing a semiconductor device.

FIGS. 12A to 12C are cross-sectional views which illustrate a method formanufacturing a semiconductor device.

FIGS. 13A to 13C are cross-sectional views which illustrate a method formanufacturing a semiconductor device.

FIGS. 14A to 14C are cross-sectional views which illustrate a method formanufacturing a semiconductor device.

FIGS. 15A and 15B are schematic views illustrating diffusion paths ofoxygen or excess oxygen diffused into an oxide semiconductor film.

FIG. 16 shows measured XRD spectra.

FIGS. 17A and 17B are TEM images of samples and FIGS. 17C to 17L areelectron diffraction patterns thereof.

FIGS. 18A to 18C show EDX mapping images of a sample.

FIGS. 19A and 19B are cross-sectional HAADF-STEM images of compositeoxide semiconductors.

FIG. 20 is a schematic cross-sectional view illustrating a compositeoxide semiconductor.

FIGS. 21A to 21C illustrate an atomic ratio of a composite oxidesemiconductor.

FIGS. 22A and 22B illustrate a sputtering apparatus.

FIG. 23 is a flowchart illustrating a method for manufacturing acomposite oxide semiconductor.

FIGS. 24A to 24C show a cross section of the vicinity of a target.

FIG. 25 is a top view illustrating one mode of a display device.

FIG. 26 is a cross-sectional view illustrating one mode of a displaydevice.

FIG. 27 is a cross-sectional view illustrating one mode of a displaydevice.

FIG. 28 illustrates a structure example of a display panel.

FIG. 29 illustrates a structure example of a display panel.

FIGS. 30A to 30C are a block diagram and circuit diagrams illustrating adisplay device.

FIG. 31 illustrates a display module.

FIGS. 32A to 32E illustrate electronic devices.

FIGS. 33A to 33G illustrate electronic devices.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments will be described with reference to drawings.However, the embodiments can be implemented in many different modes, andit will be readily appreciated by those skilled in the art that modesand details thereof can be changed in various ways without departingfrom the spirit and scope of the present invention. Thus, the presentinvention should not be interpreted as being limited to the followingdescription of the embodiments.

In the drawings, the size, the layer thickness, or the region isexaggerated for clarity in some cases. Therefore, embodiments of thepresent invention are not limited to such a scale. Note that thedrawings are schematic views showing ideal examples, and embodiments ofthe present invention are not limited to shapes or values shown in thedrawings.

Note that in this specification, ordinal numbers such as “first”,“second”, and “third” are used in order to avoid confusion amongcomponents, and the terms do not limit the components numerically.

Note that in this specification, terms for describing arrangement, suchas “over”, “above”, “under”, and “below”, are used for convenience indescribing a positional relation between components with reference todrawings. Further, the positional relation between components is changedas appropriate in accordance with a direction in which the componentsare described. Thus, the positional relation is not limited to thatdescribed with a term used in this specification and can be explainedwith another term as appropriate depending on the situation.

In this specification and the like, a transistor is an element having atleast three terminals of a gate, a drain, and a source. In addition, thetransistor has a channel region between a drain (a drain terminal, adrain region, or a drain electrode) and a source (a source terminal, asource region, or a source electrode), and current can flow between thesource and the drain through the channel region. Note that in thisspecification and the like, a channel region refers to a region throughwhich current mainly flows.

Further, functions of a source and a drain might be switched whentransistors having different polarities are employed or a direction ofcurrent flow is changed in circuit operation, for example. Therefore,the terms “source” and “drain” can be switched in this specification andthe like.

Note that in this specification and the like, the expression“electrically connected” includes the case where components areconnected through an “object having any electric function”. There is noparticular limitation on an “object having any electric function” aslong as electric signals can be transmitted and received betweencomponents that are connected through the object. Examples of an “objecthaving any electric function” are a switching element such as atransistor, a resistor, an inductor, a capacitor, and elements with avariety of functions as well as an electrode and a wiring.

In this specification and the like, the term “parallel” means that theangle formed between two straight lines is greater than or equal to −10°and less than or equal to 10°, and accordingly also covers the casewhere the angle is greater than or equal to −5° and less than or equalto 50. The term “perpendicular” means that the angle formed between twostraight lines is greater than or equal to 80° and less than or equal to100°, and accordingly also covers the case where the angle is greaterthan or equal to 85° and less than or equal to 95°.

In this specification and the like, the terms “film” and “layer” can beinterchanged with each other. For example, the term “conductive layer”can be changed into the term “conductive film” in some cases. Also, theterm “insulating film” can be changed into the term “insulating layer”in some cases.

Unless otherwise specified, the off-state current in this specificationand the like refers to a drain current of a transistor in the off state(also referred to as non-conduction state and cutoff state). Unlessotherwise specified, the off state of an n-channel transistor means thata gate-source voltage V_(gs) is lower than the threshold voltage V_(th),and the off state of a p-channel transistor means that the gate-sourcevoltage V_(gs) is higher than the threshold voltage V_(th). For example,the off-state current of an n-channel transistor sometimes refers to adrain current that flows when the gate-source voltage V_(gs) is lowerthan the threshold voltage V_(th).

The off-state current of a transistor depends on V_(gs) in some cases.Thus, “the off-state current of a transistor is lower than or equal toI” may mean “there is V_(gs) with which the off-state current of thetransistor becomes lower than or equal to I”. Furthermore, “theoff-state current of a transistor” means “the off-state current in anoff state at predetermined V_(gs)”, “the off-state current in an offstate at V_(gs) in a predetermined range”, “the off-state current in anoff state at V_(gs) with which sufficiently reduced off-state current isobtained”, or the like.

As an example, the assumption is made of an n-channel transistor wherethe threshold voltage V_(th) is 0.5 V and the drain current is 1×10⁻⁹ Aat V_(gs) of 0.5 V, 1×10⁻¹³ A at V_(gs) of 0.1 V, 1×10⁻¹⁹ A at V_(gs) of−0.5 V, and 1×10⁻²² A at V_(gs) of −0.8 V. The drain current of thetransistor is 1×10⁻¹⁹ A or lower at V_(gs) of −0.5 V or at V_(gs) in therange of −0.8 V to −0.5 V; therefore, it may be said that the off-statecurrent of the transistor is 1×10⁻¹⁹ A or lower. Since there is V_(gs)at which the drain current of the transistor is 1×10⁻²² A or lower, itmay be said that the off-state current of the transistor is 1×10⁻²² A orlower.

In this specification and the like, the off-state current of atransistor with a channel width W is sometimes represented by a currentvalue in relation to the channel width W or by a current value per givenchannel width (e.g., 1 μm). In the latter case, the off-state currentmay be expressed in the unit with the dimension of current per length(e.g., A/μm).

The off-state current of a transistor depends on temperature in somecases. Unless otherwise specified, the off-state current in thisspecification may be an off-state current at room temperature, 60° C.,85° C., 95° C., or 125° C. Alternatively, the off-state current may bean off-state current at a temperature at which the reliability requiredin a semiconductor device or the like including the transistor isensured or a temperature at which the semiconductor device or the likeincluding the transistor is used (e.g., temperature in the range of 5°C. to 35° C.). The description “an off-state current of a transistor islower than or equal to I” may refer to a situation where there is V_(gs)at which the off-state current of a transistor is lower than or equal toI at room temperature, 60° C., 85° C., 95° C., 125° C., a temperature atwhich the reliability required in a semiconductor device or the likeincluding the transistor is ensured, or a temperature at which thesemiconductor device or the like including the transistor is used (e.g.,temperature in the range of 5° C. to 35° C.).

The off-state current of a transistor depends on a drain-source voltageV_(ds) in some cases. Unless otherwise specified, the off-state currentin this specification may be an off-state current at V_(ds) of 0.1 V,0.8 V, 1 V, 1.2 V, 1.8 V, 2.5 V, 3 V, 3.3 V, 10 V, 12 V, 16 V, or 20 V.Alternatively, the off-state current might be an off-state current atV_(ds) at which the required reliability of a semiconductor device orthe like including the transistor is ensured or V_(ds) at which thesemiconductor device or the like including the transistor is used. Thedescription “an off-state current of a transistor is lower than or equalto I” may refer to a situation where there is V_(gs) at which theoff-state current of a transistor is lower than or equal to I at V_(ds)of 0.1 V, 0.8 V, 1 V, 1.2 V, 1.8 V, 2.5 V, 3 V, 3.3 V, 10 V, 12 V, 16 V,or 20 V, V_(ds) at which the required reliability of a semiconductordevice or the like including the transistor is ensured, or V_(ds) atwhich the semiconductor device or the like including the transistor isused.

In the above description of off-state current, a drain may be replacedwith a source. That is, the off-state current sometimes refers to acurrent that flows through a source of a transistor in the off state.

In this specification and the like, the term “leakage current” sometimesexpresses the same meaning as off-state current. In this specificationand the like, the off-state current sometimes refers to a current thatflows between a source and a drain when a transistor is off, forexample.

In this specification and the like, the threshold voltage of atransistor refers to a gate voltage (V_(g)) at which a channel is formedin the transistor. Specifically, in a graph where the horizontal axisrepresents the gate voltage (V_(g)) and the vertical axis represents thesquare root of drain current (I_(d)), the threshold voltage of atransistor may refer to a gate voltage (V_(g)) at the intersection ofthe square root of drain current (I_(d)) of 0 (I_(d)=0 A) and anextrapolated straight line that is tangent with the highest inclinationto a plotted curve (V_(g)−√I_(d) characteristics). Alternatively, thethreshold voltage of a transistor may refer to a gate voltage (V_(g)) atwhich the value of I_(d) [A]×L [μm]/W [μm] is 1×10⁻⁹ [A] where L ischannel length and W is channel width.

In this specification and the like, a “semiconductor” can havecharacteristics of an “insulator” when the conductivity is sufficientlylow, for example. Further, a “semiconductor” and an “insulator” cannotbe strictly distinguished from each other in some cases because a borderbetween the “semiconductor” and the “insulator” is not clear.Accordingly, a “semiconductor” in this specification and the like can becalled an “insulator” in some cases. Similarly, an “insulator” in thisspecification and the like can be called a “semiconductor” in somecases. An “insulator” in this specification and the like can be called a“semi-insulator” in some cases.

In this specification and the like, a “semiconductor” can havecharacteristics of a “conductor” when the conductivity is sufficientlyhigh, for example. Further, a “semiconductor” and a “conductor” cannotbe strictly distinguished from each other in some cases because a borderbetween the “semiconductor” and the “conductor” is not clear.Accordingly, a “semiconductor” in this specification and the like can becalled a “conductor” in some cases. Similarly, a “conductor” in thisspecification and the like can be called a “semiconductor” in somecases.

In this specification and the like, an impurity in a semiconductorrefers to an element that is not a main component of a semiconductorfilm. For example, an element with a concentration of lower than 0.1atomic % is an impurity. If a semiconductor contains an impurity, thedensity of states (DOS) may be formed therein, the carrier mobility maybe decreased, or the crystallinity may be decreased, for example. In thecase where the semiconductor includes an oxide semiconductor, examplesof the impurity which changes the characteristics of the semiconductorinclude Group 1 elements, Group 2 elements, Group 13 elements, Group 14elements, Group 15 elements, and transition metals other than the maincomponents; specific examples are hydrogen (also included in water),lithium, sodium, silicon, boron, phosphorus, carbon, and nitrogen. Whenthe semiconductor is an oxide semiconductor, oxygen vacancies may beformed by entry of impurities such as hydrogen, for example.Furthermore, in the case where the semiconductor includes silicon,examples of the impurity which changes the characteristics of thesemiconductor include oxygen, Group 1 elements except oxygen andhydrogen, Group 2 elements, Group 13 elements, and Group 15 elements.

Note that in this specification and the like, “In:Ga:Zn=4:2:3 or aneighborhood of In:Ga:Zn=4:2:3” refers to an atomic ratio where, when Inis 4 with respect to the total number of atoms, Ga is greater than orequal to 1 and less than or equal to 3 (1≤Ga≤3) and Zn is greater thanor equal to 2 and less than or equal to 4 (2≤Zn≤4). “In:Ga:Zn=5:1:6 or aneighborhood of In:Ga:Zn=5:1:6” refers to an atomic ratio where, when Inis 5 with respect to the total number of atoms, Ga is greater than 0.1and less than or equal to 2 (0.1<Ga≤2) and Zn is greater than or equalto 5 and less than or equal to 7 (5≤Zn≤7). “In:Ga:Zn=1:1:1 or aneighborhood of In:Ga:Zn=1:1:1” refers to an atomic ratio where, when Inis 1 with respect to the total number of atoms, Ga is greater than 0.1and less than or equal to 2 (0.1<Ga≤2) and Zn is greater than 0.1 andless than or equal to 2 (0.1<Zn≤2).

Embodiment 1

In this embodiment, a semiconductor device of one embodiment of thepresent invention and a manufacturing method thereof are described withreference to FIG. 1, FIG. 2, FIG. 3, and FIGS. 4A to 4C.

One embodiment of the present invention is a manufacturing method of asemiconductor device including a first step of forming a first oxidesemiconductor film over a substrate, a second step of heating the firstoxide semiconductor film, and a third step of forming a second oxidesemiconductor film over the first oxide semiconductor film. The first tothird steps are performed in an atmosphere in which water vapor partialpressure is lower than water vapor partial pressure in atmospheric air,and the first step, the second step, and the third step are successivelyperformed in this order.

In the case where a plurality of oxide semiconductor films (here, afirst oxide semiconductor film and a second oxide semiconductor film)are stacked, impurities (specifically, hydrogen, moisture, and the like)at the interface between the first oxide semiconductor film and thesecond oxide semiconductor film become a problem.

Attachment or entry of impurities to or into the interface between thefirst oxide semiconductor film and the second oxide semiconductor filmwill reduce the reliability of the semiconductor device in some cases.Therefore, the amount of impurities such as hydrogen or moisture at theinterface between the first oxide semiconductor film and the secondoxide semiconductor film is preferably as small as possible.

In one embodiment of the present invention, the first step of formingthe first oxide semiconductor film, the second step of heating the firstoxide semiconductor film, and the third step of forming the second oxidesemiconductor film over the first oxide semiconductor film are performedin an atmosphere in which water vapor partial pressure is lower thanwater vapor partial pressure in atmospheric air, and the first to thethird steps are performed successively.

Another step may be performed between the above steps. For example, theanother step can be performed between the first step and the secondstep. Specific examples of the another step include a step oftransferring the substrate. It is also preferable that the another stepbe performed in an atmosphere in which water vapor partial pressure islower than water vapor partial pressure in atmospheric air.

Note that the atmosphere in which water vapor partial pressure is lowerthan water vapor partial pressure in atmospheric air corresponds to anatmosphere in which pressure is lower than at least pressure ofatmospheric air. Specifically, the pressure in the atmosphere is set toa low vacuum or a medium vacuum (several hundreds of pascals to 0.1pascals) or a high vacuum or an ultra-high vacuum (0.1 Pa to 1×10⁻⁷ Pa).

By the above embodiment, it is possible to inhibit attachment or entryof impurities to or into the interface between the first oxidesemiconductor film and the second oxide semiconductor film. Hydrogen,moisture, or the like that might be contained in the first oxidesemiconductor film can be removed by performing the second step. Notethat in this specification and the like, treatment for removing hydrogencontained in an oxide semiconductor film is referred to asdehydrogenation treatment in some cases. Similarly, treatment forremoving moisture contained in an oxide semiconductor film is referredto as dehydration treatment in some cases.

By the above embodiment, the plurality of oxide semiconductor films caneach be an oxide semiconductor film in which the impurity concentrationis low and the density of defect states is low.

Note that it is preferable to use, as the oxide semiconductor film, anoxide semiconductor film in which the impurity concentration is low anddensity of defect states is low, in which case the transistors can havemore excellent electrical characteristics. Here, the state in whichimpurity concentration is low and density of defect states is low (thenumber of oxygen vacancies is small) is referred to as “highly purifiedintrinsic” or “substantially highly purified intrinsic”. Note thatimpurities in an oxide semiconductor film are typically water, hydrogen,and the like. In this specification and the like, reducing or removingwater and hydrogen from the oxide semiconductor film is referred to asdehydration or dehydrogenation in some cases. Moreover, adding oxygen tothe oxide semiconductor film is referred to as oxygen addition in somecases, and a state in which oxygen in excess of the stoichiometriccomposition is contained is referred to as an oxygen-excess state insome cases.

A highly purified intrinsic or substantially highly purified intrinsicoxide semiconductor film has few carrier generation sources, and thuscan have a low carrier density. Thus, a transistor in which a channelregion is formed in the oxide semiconductor film rarely has a negativethreshold voltage (is rarely normally on). A highly purified intrinsicor substantially highly purified intrinsic oxide semiconductor film hasa low density of defect states and accordingly has a low density of trapstates in some cases. Further, the highly purified intrinsic orsubstantially highly purified intrinsic oxide semiconductor film has anextremely low off-state current; even when an element has a channelwidth of 1×10⁶ μm and a channel length (L) of 10 μm, the off-statecurrent can be less than or equal to the measurement limit of asemiconductor parameter analyzer, i.e., less than or equal to 1×10⁻¹³ A,at a voltage (drain voltage) between a source electrode and a drainelectrode of from 1 V to 10 V.

<1-1. Manufacturing Method of Semiconductor Device>

Next, a manufacturing method of a semiconductor device of one embodimentof the present invention will be described with reference to FIG. 1 andFIG. 2. Note that FIG. 1 and FIG. 2 are flow charts each showing themanufacturing method of a semiconductor device of one embodiment of thepresent invention.

[First Step: Formation of First Oxide Semiconductor Film]

The first step is a step of forming the first oxide semiconductor filmover a substrate (see Step S101 in FIG. 1).

Note that although the step of forming the first oxide semiconductorfilm over the substrate is described as an example in this embodiment,the step is not limited thereto. For example, the first oxidesemiconductor film may be formed over any of a variety of films such asan insulating film, a semiconductor film, and a conductive film which isformed over a substrate.

The first oxide semiconductor film preferably includes In, M (M is Ga,Al, Y, or Sn) and Zn. The first oxide semiconductor film preferablyincludes a region in which the atomic proportion of In is larger thanthe atomic proportion of M. For example, the atomic ratio of In to M andZn in the first oxide semiconductor film is preferably In:M:Zn=4:2:3 ora neighborhood of In:M:Zn=4:2:3, or In:M:Zn=5:1:7 or a neighborhood ofIn:M:Zn=5:1:7.

As a gas used to form the first oxide semiconductor film, at least oneof an inert gas (typically, argon) and an oxygen gas is used.

For example, one of an argon gas and an oxygen gas is used to form thefirst oxide semiconductor film. Note that the proportion of the flowrate of the oxygen gas to the flow rate of a whole gas (the proportionis also referred to as an oxygen flow rate ratio) at the time of formingthe first oxide semiconductor film is higher than or equal to 0% andlower than or equal to 30%, preferably higher than or equal to 5% andlower than or equal to 15%. With the oxygen flow rate ratio in the aboverange, the first oxide semiconductor film can have low crystallinity.Furthermore, the oxygen flow rate ratio in the above range enables thematerial composition of the first oxide semiconductor film to be aCAC-OS described later.

The substrate temperature at the time of forming the first oxidesemiconductor film is set higher than or equal to room temperature (25°C.) and lower than or equal to 200° C., preferably higher than or equalto room temperature and lower than or equal to 130° C. The substratetemperature in the above range can prevent bending or warpage of thesubstrate in the case where the substrate is a large glass substrate.

[Second Step: Heating of First Oxide Semiconductor Film]

The second step is a step of heating the first oxide semiconductor film(see Step S201 in FIG. 1)

In the case where the first oxide semiconductor film is formed withoutperforming heating intentionally (e.g., at room temperature), forexample, impurities such as moisture or hydrogen can enter the film orcan be attached to the surface of the film during or after deposition.In that case, impurities such as moisture or hydrogen can be removedfrom inside the first oxide semiconductor film or the surface of thefirst oxide semiconductor film by performing a heating step after thefirst oxide semiconductor film is formed.

The temperature for heating the first oxide semiconductor film is higherthan or equal to 100° C. and lower than or equal to 450° C., preferablyhigher than or equal to 100° C. and lower than or equal to 350° C.,further preferably higher than or equal to 150° C. and lower than orequal to 250° C. The treatment time of the step for heating the firstoxide semiconductor film is longer than or equal to 1 minute and shorterthan or equal to 60 minutes, preferably longer than or equal to 2minutes and shorter than or equal to 30 minutes, further preferablylonger than or equal to 3 minutes and shorter than or equal to 5minutes. Since the treatment time in the heating step affects throughputof the deposition apparatus, the practitioner can select suitable timeas appropriate.

[Third Step: Formation of Second Oxide Semiconductor Film]

The third step is a step of forming the second oxide semiconductor filmover the first oxide semiconductor film (see Step S301 in FIG. 1).

The second oxide semiconductor film preferably includes In, M (M is Ga,Al, Y, or Sn) and Zn. The second oxide semiconductor film preferablyincludes a region in which the atomic proportion of In is larger thanthe atomic proportion of M. For example, the atomic ratio of In to M andZn in the second oxide semiconductor film is preferably In:M:Zn=4:2:3 ora neighborhood of In:M:Zn=4:2:3, or In:M:Zn=5:1:7 or a neighborhood ofIn:M:Zn=5:1:7.

As a gas used to form the second oxide semiconductor film, at least oneof an inert gas (typically, argon) and an oxygen gas is used.

For example, one of an argon gas and an oxygen gas is used to form thesecond oxide semiconductor film. The oxygen flow rate ratio at the timeof forming the second oxide semiconductor film is higher than 30% andlower than or equal to 100%, preferably higher than or equal to 50% andlower than or equal to 100%, further preferably higher than or equal to70% and lower than or equal to 100%. With the oxygen flow rate ratio inthe above range, the second oxide semiconductor film can have highcrystallinity.

The substrate temperature at the time of forming the second oxidesemiconductor film is set higher than or equal to room temperature (25°C.) and lower than or equal to 200° C., preferably higher than or equalto room temperature and lower than or equal to 130° C. The substratetemperature in the above range can prevent bending or warpage of thesubstrate in the case where the substrate is a large glass substrate.

When the compositions of the first oxide semiconductor film and thesecond oxide semiconductor film are substantially the same, they can beformed using the same sputtering target and the manufacturing cost canbe thus reduced. Since the same sputtering target is used, the firstoxide semiconductor film and the second oxide semiconductor film can beformed successively in the same deposition apparatus in vacuum. This cansuppress entry of impurities into the interface between the first oxidesemiconductor film and the second oxide semiconductor film. Since theheat treatment is performed after the first oxide semiconductor film isformed in one embodiment of the present invention, impurities at theinterface between the first oxide semiconductor film and the secondoxide semiconductor film can be further reduced.

The first to third steps are successively performed in an atmosphere inwhich the water vapor partial pressure is lower than the water vaporpartial pressure in atmospheric air, and the first step, the secondstep, and the third step are successively performed in this order.

Before the formation of the first oxide semiconductor film (Step S101),heat treatment may be performed on the substrate (Step S401 in FIG. 2).

The fourth step is a step for heating the substrate. Water or the likeabsorbed on the substrate can be suitably removed by performing thefourth step. For example, when the first oxide semiconductor film isformed in a state where water or the like is absorbed on the surface ofthe substrate, moisture or the like enters the first oxide semiconductorfilm, which affects the transistor characteristics or the like.

In the case of performing the fourth step, the fourth step, the firststep, the second step, and the third step are successively performed inthis order as shown in FIG. 2. The first to fourth steps are performedin an atmosphere in which the water vapor partial pressure is lower thanthe water vapor partial pressure in atmospheric air.

<1-2. Structure Example of Deposition Apparatus>

Here, a structure example of a deposition apparatus which can be usedfor forming the semiconductor device of one embodiment is described withreference to FIG. 3 and FIGS. 4A to 4C.

When the deposition apparatus shown in FIG. 3 and FIGS. 4A to 4C isused, impurities which can enter the oxide semiconductor film(especially hydrogen and water) can be reduced.

FIG. 3 is a top view schematically illustrating a single wafermulti-chamber deposition apparatus 4000. The deposition apparatus 4000includes an atmosphere-side substrate supply chamber 4001 including acassette port 4101 for storing substrates and an alignment port 4102 forperforming alignment of substrates, an atmosphere-side substratetransfer chamber 4002 through which a substrate is transferred from theatmosphere-side substrate supply chamber 4001, a load lock chamber 4003a where a substrate is carried in and the pressure is switched fromatmospheric pressure to reduced pressure or from reduced pressure toatmospheric pressure, an unload lock chamber 4003 b where a substrate iscarried out and the pressure is switched from reduced pressure toatmospheric pressure or from atmospheric pressure to reduced pressure, atransfer chamber 4004 where a substrate is transferred in a vacuum, asubstrate heating chamber 4005 where a substrate is heated, anddeposition chambers 4006 a, 4006 b, and 4006 c in each of which asputtering target is placed for deposition.

Note that a plurality of the cassette ports 4101 may be provided asillustrated in FIG. 3 (in FIG. 3, three cassette ports 4101 areprovided).

The atmosphere-side substrate transfer chamber 4002 is connected to theload lock chamber 4003 a and the unload lock chamber 4003 b, the loadlock chamber 4003 a and the unload lock chamber 4003 b are connected tothe transfer chamber 4004, and the transfer chamber 4004 is connected tothe substrate heating chamber 4005 and the deposition chambers 4006 a,4006 b, and 4006 c.

Note that gate valves 4104 are provided in connecting portions betweenthe chambers so that each chamber excluding the atmosphere-sidesubstrate supply chamber 4001 and the atmosphere-side substrate transferchamber 4002 can be independently kept in a vacuum state. Moreover, theatmosphere-side substrate transfer chamber 4002 and the transfer chamber4004 each include a transfer robot 4103, with which a glass substratecan be transferred.

Further, it is preferable that the substrate heating chamber 4005 alsoserve as a plasma treatment chamber. In the deposition apparatus 4000,substrates can be transferred without being exposed to the air betweentreatments, and adsorption of impurities to substrates can besuppressed. In addition, the order of film formation, heat treatment, orthe like can be freely determined. Note that the number of transferchambers, the number of deposition chambers, the number of load lockchambers, the number of unload lock chambers, and the number ofsubstrate heating chambers are not limited to the above, and the numbersthereof can be set as appropriate depending on the space forinstallation or the process conditions.

FIG. 4A, FIG. 4B, and FIG. 4C are a cross-sectional view taken alongdashed-dotted line A1-A2, a cross-sectional view taken alongdashed-dotted line B1-B2, and a cross-sectional view taken alongdashed-dotted line B2-B3, respectively, in the deposition apparatus 4000illustrated in FIG. 3.

FIG. 4A is a cross-sectional view of the substrate heating chamber 4005and the transfer chamber 4004. The substrate heating chamber 4005 shownin FIG. 4A includes a plurality of heating stages 4105 which can hold asubstrate.

Note that although the substrate heating chamber 4005 including theseven heating stages 4105 is illustrated in FIG. 4A, one embodiment ofthe present invention is not limited to such a structure. The number ofheating stages 4105 may be greater than or equal to one and less thanseven. Alternatively, the number of heating stages 4105 may be greaterthan or equal to eight. It is preferable to increase the number of theheating stages 4105 because a plurality of substrates can be subjectedto heat treatment at the same time, which leads to an increase inproductivity. Further, the substrate heating chamber 4005 is connectedto a vacuum pump 4200 through a valve. As the vacuum pump 4200, a drypump and a mechanical booster pump can be used, for example.

As a heating mechanism which can be used for the substrate heatingchamber 4005, a resistance heater or the like may be used for heating,for example. Alternatively, heat conduction or heat radiation from amedium such as a heated gas may be used as the heating mechanism. Forexample, a rapid thermal annealing (RTA) apparatus such as a gas rapidthermal annealing (GRTA) apparatus or a lamp rapid thermal annealing(LRTA) apparatus can be used. The LRTA apparatus is an apparatus forheating an object by radiation of light (an electromagnetic wave)emitted from a lamp such as a halogen lamp, a metal halide lamp, a xenonarc lamp, a carbon arc lamp, a high-pressure sodium lamp, or ahigh-pressure mercury lamp. In the GRTA apparatus, heat treatment isperformed using a high-temperature gas. An inert gas is used as a gas.

Moreover, the substrate heating chamber 4005 is connected to a refiner4301 through a mass flow controller 4300. Note that although the massflow controller 4300 and the refiner 4301 can be provided for each of aplurality of kinds of gases, only one mass flow controller 4300 and onerefiner 4301 are provided for simplicity. As the gas introduced to thesubstrate heating chamber 4005, a gas whose dew point is −80° C. orlower, preferably −100° C. or lower can be used; for example, an oxygengas, a nitrogen gas, and a rare gas (e.g., an argon gas) are used.

The transfer chamber 4004 includes the transfer robot 4103. The transferrobot 4103 includes a plurality of movable portions and an arm forholding a substrate and can transfer a substrate to each chamber.Further, the transfer chamber 4004 is connected to a vacuum pump 4200and a cryopump 4201 through valves. With such a structure, evacuation isperformed inside the transfer chamber 4004 using the vacuum pump 4200from the atmospheric pressure to a low or medium vacuum (approximatelyseveral hundred pascals to 0.1 pascals) and then the valves are switchedand evacuation is performed using the cryopump 4201 from the mediumvacuum to a high or ultrahigh vacuum (approximately 0.1 Pa to 1×10⁻⁷Pa).

Alternatively, two or more cryopumps 4201 may be connected in parallelto the transfer chamber 4004. With such a structure, even when one ofthe cryopumps is in regeneration, exhaust can be performed using any ofthe other cryopumps. Note that the above regeneration refers totreatment for discharging molecules (or atoms) entrapped in a cryopump.When molecules (or atoms) are entrapped too much in a cryopump, theexhaust capability of the cryopump is lowered; therefore, regenerationis performed regularly.

FIG. 4B illustrates a cross section of the deposition chamber 4006 b,the transfer chamber 4004, and the load lock chamber 4003 a. Here, thedetails of the deposition chamber (sputtering chamber) are describedwith reference to FIG. 4B.

The deposition chamber 4006 b illustrated in FIG. 4B includes a target4106, an attachment protection plate 4107, and a substrate stage 4108.Note that the substrate stage 4108 here is provided with a substrate4109. Although not illustrated, the substrate stage 4108 may be providedwith a substrate holding mechanism for holding the substrate 4109, aback side heater for heating the substrate 4109 from the back side, orthe like.

Note that the substrate stage 4108 is held substantially vertically to afloor during deposition and is held substantially parallel to the floorwhen the substrate is delivered. In FIG. 4B, the position where thesubstrate stage 4108 is held when the substrate is delivered is denotedby a dashed line. With such a structure, the probability that dust or aparticle which might be entered at the time of the film formation isattached to the substrate 4109 can be suppressed as compared with thecase where the substrate stage 4108 is held parallel to the floor.However, there is a possibility that the substrate 4109 falls when thesubstrate stage 4108 is held vertically (90°) to the floor; therefore,the angle of the substrate stage 4108 to the floor is preferably widerthan or equal to 80° and narrower than 90°.

The deposition plate 4107 can prevent sputtered particles separated fromthe target 4106 from being deposited on a region where deposition is notnecessary. Moreover, the deposition plate 4107 is preferably processedto prevent accumulated sputtered particles from being separated. Forexample, blasting treatment which increases surface roughness may beperformed, or roughness may be formed on the surface of the attachmentprotection plate 4107.

The deposition chamber 4006 b is connected to a mass flow controller4300 via a gas heating mechanism 4302, and the gas heating mechanism4302 is connected to a refiner 4301 via the mass flow controller 4300.With the gas heating system 4302, a gas which is introduced to thedeposition chamber 4006 b can be heated to a temperature higher than orequal to 40° C. and lower than or equal to 400° C., preferably higherthan or equal to 50° C. and lower than or equal to 200° C. Note thatalthough the gas heating mechanism 4302, the mass flow controller 4300,and the refiner 4301 can be provided for each of a plurality of kinds ofgases, only one gas heating mechanism 4302, one mass flow controller4300, and one refiner 4301 are provided for simplicity. As the gasintroduced to the deposition chamber 4006 b, a gas whose dew point is−80° C. or lower, preferably −100° C. or lower can be used; for example,an oxygen gas, a nitrogen gas, and a rare gas (e.g., an argon gas) areused.

The deposition chamber 4006 b is connected to a turbo molecular pump4202 and the vacuum pump 4200 through valves.

In addition, the deposition chamber 4006 b is provided with a cryotrap4110.

The cryotrap 4110 is a mechanism that can adsorb a molecule (or an atom)having a relatively high melting point, such as water. The turbomolecular pump 4202 is capable of stably removing a large-sized molecule(or atom), needs low frequency of maintenance, and thus enables highproductivity; on the other hand, it has a low capability in removinghydrogen and water. Hence, the cryotrap 4110 is connected to thedeposition chamber 4006 b so as to have a high capability in removingwater or the like. The temperature of a refrigerator of the cryotrap4110 is set to be lower than or equal to 100 K, preferably lower than orequal to 80 K. In the case where the cryotrap 4110 includes a pluralityof refrigerators, it is preferable to set the temperatures of therefrigerators at different temperatures because efficient exhaust ispossible. For example, the temperature of a first-stage refrigerator maybe set to be lower than or equal to 100 K and the temperature of asecond-stage refrigerator may be set to be lower than or equal to 20 K.

Note that the exhaust method of the deposition chamber 4006 b is notlimited to the above, and a structure similar to that in the exhaustmethod described above for the transfer chamber 4004 (the exhaust methodusing the cryopump and the vacuum pump) may be employed. Needless tosay, the exhaust method of the transfer chamber 4004 may have astructure similar to that of the deposition chamber 4006 b (the exhaustmethod using the turbo molecular pump and the vacuum pump).

Note that in each of the above transfer chamber 4004, the substrateheating chamber 4005, and the deposition chamber 4006 b, the backpressure (total pressure) and the partial pressure of each gas molecule(atom) are preferably set as follows. In particular, the back pressureand the partial pressure of each gas molecule (atom) in the depositionchamber 4006 b need to be noted because impurities might enter a film tobe formed.

In each of the above chambers, the back pressure (total pressure) isless than or equal to 1×10⁻⁴ Pa, preferably less than or equal to 3×10⁻⁵Pa, and further preferably less than or equal to 1×10⁻⁵ Pa. In each ofthe above chambers, the partial pressure of a gas molecule (atom) havinga mass-to-charge ratio (m/z) of 18 is less than or equal to 3×10⁻⁵ Pa,preferably less than or equal to 1×10⁻⁵ Pa, and further preferably lessthan or equal to 3×10⁻⁶ Pa. Moreover, in each of the above chambers, thepartial pressure of a gas molecule (atom) having a mass-to-charge ratio(m/z) of 28 is less than or equal to 3×10⁻⁵ Pa, preferably less than orequal to 1×10⁻⁵ Pa, and further preferably less than or equal to 3×10⁻⁶Pa. Furthermore, in each of the above chambers, the partial pressure ofa gas molecule (atom) having a mass-to-charge ratio (m/z) of 44 is lessthan or equal to 3×10⁻⁵ Pa, preferably less than or equal to 1×10⁻⁵ Pa,and further preferably less than or equal to 3×10⁻⁶ Pa.

Note that a total pressure and a partial pressure in a vacuum chambercan be measured using a mass analyzer. For example, Qulee CGM-051, aquadrupole mass analyzer (also referred to as Q-mass) manufactured byULVAC, Inc. can be used.

Next, the details of the transfer chamber 4004 and the load lock chamber4003 a illustrated in FIG. 4B and the atmosphere-side substrate transferchamber 4002 and the atmosphere-side substrate supply chamber 4001illustrated in FIG. 4C are described. Note that FIG. 4C is across-sectional view of the atmosphere-side substrate transfer chamber4002 and the atmosphere-side substrate supply chamber 4001.

For the transfer chamber 4004 illustrated in FIG. 4B, the description ofthe transfer chamber 4004 illustrated in FIG. 4A can be referred to.

The load lock chamber 4003 a includes a substrate delivery stage 4111.When a pressure in the load lock chamber 4003 a becomes atmosphericpressure by being increased from reduced pressure, the substratedelivery stage 4111 receives a substrate from the transfer robot 4103provided in the atmosphere-side substrate transfer chamber 4002. Afterthat, the load lock chamber 4003 a is evacuated into vacuum so that thepressure therein becomes reduced pressure and then the transfer robot4103 provided in the transfer chamber 4004 receives the substrate fromthe substrate delivery stage 4111.

Further, the load lock chamber 4003 a is connected to a vacuum pump 4200and a cryopump 4201 through valves. For a method for connectingevacuation systems such as the vacuum pump 4200 and the cryopump 4201,the description of the method for connecting the transfer chamber 4004can be referred to, and the description thereof is omitted here. Notethat the unload lock chamber 4003 b illustrated in FIG. 3 can have astructure similar to that in the load lock chamber 4003 a.

The atmosphere-side substrate transfer chamber 4002 includes thetransfer robot 4103. The transfer robot 4103 can deliver a substratefrom the cassette port 4101 to the load lock chamber 4003 a or deliver asubstrate from the load lock chamber 4003 a to the cassette port 4101.Furthermore, a mechanism for suppressing entry of dust or a particle,such as a high-efficiency particulate air (HEPA) filter, may be providedabove the atmosphere-side substrate transfer chamber 4002 and theatmosphere-side substrate supply chamber 4001.

The atmosphere-side substrate supply chamber 4001 includes a pluralityof cassette ports 4101. The cassette port 4101 can hold a plurality ofsubstrates.

The oxide semiconductor film is formed with the use of the abovedeposition apparatus, so that the entry of impurities into the oxidesemiconductor film can be suppressed. Further, when a film in contactwith the oxide semiconductor film is formed with the use of the abovedeposition apparatus, the entry of impurities into the oxidesemiconductor film from the film in contact therewith can be suppressed.

For example, with the deposition apparatus shown in FIG. 3 and FIGS. 4Ato 4C, the semiconductor device of one embodiment of the presentinvention can be manufactured in the following order.

The first oxide semiconductor film is formed in a deposition chamber4006 a. Then, the first oxide semiconductor film is heated in thesubstrate heating chamber 4005. Then, the second oxide semiconductorfilm is formed in the deposition chamber 4006 b.

Alternatively, the substrate is heated in the substrate heating chamber4005. The first oxide semiconductor film is formed in the depositionchamber 4006 a. Then, the first oxide semiconductor film is heated inthe substrate heating chamber 4005. Next, the second oxide semiconductorfilm is formed in the deposition chamber 4006 b. Note that by usingdifferent oxygen gas flow rates for forming the first oxidesemiconductor film and the second oxide semiconductor film as describedabove, the crystallinity or the material composition of the first oxidesemiconductor film can be made different from the crystallinity or thematerial composition of the second oxide semiconductor film.

Note that a method for forming the first oxide semiconductor film andthe second oxide semiconductor film in different deposition chambers isdescribed in the above example; however, the present invention is notlimited thereto. For example, the first oxide semiconductor film and thesecond oxide semiconductor film may be formed in the same depositionchamber. In the case where a substrate-heating unit is provided in thedeposition chamber, the formation of the first oxide semiconductor film,the heating of the first oxide semiconductor film, and the formation ofthe second oxide semiconductor film are performed in the same depositionchamber.

At least part of this embodiment can be implemented in combination withany of the other embodiments and the other examples described in thisspecification as appropriate.

Embodiment 2

In this embodiment, a semiconductor device of one embodiment of thepresent invention and a manufacturing method thereof are described withreference to FIGS. 5A to 5C to FIGS. 15A and 15B.

<2-1. Structure Example 1 of Semiconductor Device 1>

FIG. 5A is a top view of a transistor 100 that is a semiconductor deviceof one embodiment of the present invention. FIG. 5B is a cross-sectionalview taken along a dashed dotted line X1-X2 in FIG. 5A. FIG. 5C is across-sectional view taken along a dashed dotted line Y1-Y2 in FIG. 5A.Note that in FIG. 5A, some components of the transistor 100 (e.g., aninsulating film serving as a gate insulating film) are not illustratedto avoid complexity. Furthermore, the direction of the dashed dottedline X1-X2 may be referred to as a channel length direction, and thedirection of the dashed dotted line Y1-Y2 may be referred to as achannel width direction. As in FIG. 5A, some components are notillustrated in some cases in top views of transistors described below.

The transistor 100 includes a conductive film 104 over a substrate 102,an insulating film 106 over the substrate 102 and the conductive film104, an oxide semiconductor film 108 over the insulating film 106, aconductive film 112 a over the oxide semiconductor film 108, and aconductive film 112 b over the oxide semiconductor film 108.Furthermore, an insulating film 114, an insulating film 116 over theinsulating film 114, and an insulating film 118 over the insulating film116 are formed over the transistor 100, specifically over the oxidesemiconductor film 108, the conductive film 112 a, and the conductivefilm 112 b.

Note that the transistor 100 is what is called a channel-etchedtransistor.

Furthermore, the oxide semiconductor film 108 includes an oxidesemiconductor film 108_1 over the insulating film 106, and an oxidesemiconductor film 108_2 over the oxide semiconductor film 108_1. Notethat the oxide semiconductor film 108_1 corresponds to the first oxidesemiconductor film described in Embodiment 1, and the oxidesemiconductor film 108_2 corresponds to the second oxide semiconductorfilm described in Embodiment 1. That is, the oxide semiconductor film108_1 and the oxide semiconductor film 108_2 each independently includea region in which the atomic proportion of In is higher than the atomicproportion of M.

When the oxide semiconductor film 108_1 and the oxide semiconductor film108_2 each independently include a region in which the atomic proportionof In is higher than the atomic proportion of M, the transistor 100 canhave high field-effect mobility. Specifically, the field-effect mobilityof the transistor 100 can be higher than 50 cm²/Vs, preferably higherthan 100 cm²/Vs.

For example, the use of the transistor with high field-effect mobilityin a gate driver that generates a gate signal allows a display device tohave a narrow frame. The use of the transistor with high field-effectmobility in a source driver (particularly in a demultiplexer connectedto an output terminal of a shift register included in a source driver)that is included in a display device and supplies a signal from a signalline can reduce the number of wirings connected to the display device.

Even when the oxide semiconductor film 108_1 and the oxide semiconductorfilm 108_2 each independently include a region in which the atomicproportion of In is higher than the atomic proportion of M, thefield-effect mobility might be low if the oxide semiconductor film 108_1and the oxide semiconductor film 108_2 have high crystallinity.

However, in this embodiment, the oxide semiconductor film 108_1 includesa region having lower crystallinity than the oxide semiconductor film108_2. Note that the crystallinity of the oxide semiconductor film 108can be determined by analysis by X-ray diffraction (XRD) or with atransmission electron microscope (TEM).

In the case where the oxide semiconductor film 108_1 has a region withlow crystallinity, the following effects can be achieved.

First, oxygen vacancies that might be formed in the oxide semiconductorfilm 108 will be described.

Oxygen vacancies formed in the oxide semiconductor film 108 adverselyaffect the transistor characteristics and therefore cause a problem. Forexample, oxygen vacancies formed in the oxide semiconductor film 108 arebonded to hydrogen to serve as a carrier supply source. The carriersupply source generated in the oxide semiconductor film 108 causes achange in the electrical characteristics, typically, shift in thethreshold voltage, of the transistor 100 including the oxidesemiconductor film 108. Therefore, it is preferable that the amount ofoxygen vacancies in the oxide semiconductor film 108 be as small aspossible.

In view of this, one embodiment of the present invention is a structurein which insulating films in the vicinity of the oxide semiconductorfilm 108, specifically the insulating films 114 and 116 formed over theoxide semiconductor film 108, include excess oxygen. Oxygen or excessoxygen is transferred from the insulating film 114 and the insulatingfilms 116 to the oxide semiconductor film 108, whereby the oxygenvacancies in the oxide semiconductor film can be reduced.

Here, the path of oxygen or excess oxygen diffused into the oxidesemiconductor film 108 will be described with reference to FIGS. 15A and15B. FIGS. 15A and 15B are schematic views illustrating the diffusionpaths of oxygen or excess oxygen diffused into the oxide semiconductorfilm 108. FIG. 15A is the schematic view in the channel length directionand FIG. 15B is that in the channel width direction.

Oxygen or excess oxygen of the insulating films 114 and 116 is diffusedto the oxide semiconductor film 108_1 from above, i.e., through theoxide semiconductor film 108_2 (Route 1 in FIGS. 15A and 15B).

In another case, oxygen or excess oxygen of the insulating films 114 and116 is diffused into the oxide semiconductor film 108 through the sidesurfaces of the oxide semiconductor film 108_1 and the oxidesemiconductor film 108_2 (Route 2 in FIG. 15B).

For example, diffusion of oxygen or excess oxygen by Route 1 shown inFIGS. 15A and 15B is sometimes prevented when the oxide semiconductorfilm 108_2 has high crystallinity. In contrast, oxygen or excess oxygencan be diffused to the oxide semiconductor film 108_1 and the oxidesemiconductor film 108_2 through the side surfaces of the oxidesemiconductor film 108_1 and the oxide semiconductor film 108_2 by Route2 shown in FIG. 15B.

The oxide semiconductor film 108_1 includes a region having lowercrystallinity than the oxide semiconductor film 108_2, and the regionserves as a diffusion path of excess oxygen. Thus, excess oxygen can bediffused to the oxide semiconductor film 108_2 that have highercrystallinity than the oxide semiconductor film 108_1 by Route 2 shownin FIG. 15B. Although not shown in FIGS. 15A and 15B, when theinsulating film 106 contains oxygen or excess oxygen, the oxygen orexcess oxygen might be diffused also from the insulating film 106 intothe oxide semiconductor film 108.

Although not shown in FIGS. 15A and 15B, in the case where an oxygen gasis used at the time of forming the oxide semiconductor film 108_2, theoxygen gas can be added to the oxide semiconductor film 108_1. The oxidesemiconductor film 108_1 that has a small thickness, e.g., the oxidesemiconductor film 108_1 that has a thickness of 5 nm or more and 40 nmor less or a thickness of 10 nm or more and 20 nm or less, is suitable,in which case the oxygen gas at the time of forming the oxidesemiconductor film 108_2 can be introduced into the oxide semiconductorfilm 108_1.

As described above, a stacked-layer structure that includes the oxidesemiconductor films having different crystal structures is formed in asemiconductor device of one embodiment of the present invention and theregion with low crystallinity serves as a diffusion path of excessoxygen, whereby the semiconductor device can be highly reliable.

Note that in the case where the oxide semiconductor film 108 consistsonly of an oxide semiconductor film with low crystallinity, thereliability might be lowered because of attachment or entry ofimpurities (e.g., hydrogen or moisture) to the back channel side of theoxide semiconductor film, i.e., a region corresponding to the oxidesemiconductor film 108_2.

Impurities such as hydrogen or moisture entering the channel region ofthe oxide semiconductor film 108 adversely affect the transistorcharacteristics and therefore cause a problem. Therefore, it ispreferable that the amount of impurities such as hydrogen or moisture inthe oxide semiconductor film 108 be as small as possible.

In view of this, the crystallinity of oxide semiconductor films over theoxide semiconductor film is increased in one embodiment of the presentinvention. Owing to this, impurities that might enter the oxidesemiconductor film 108 can be suppressed. In particular, the highercrystallinity of the oxide semiconductor film 108_2 can inhibit damageat the time of processing the conductive films 112 a and 112 b. Thesurface of the oxide semiconductor film 108, i.e., the surface of theoxide semiconductor film 108_2 is exposed to an etchant or an etchinggas at the time of processing the conductive films 112 a and 112 b. Theoxide semiconductor film 108_2 has etching resistance superior to theoxide semiconductor film 108_1 owing to its region with highercrystallinity than the oxide semiconductor film 108_1. Thus, the oxidesemiconductor film 108_2 serves as an etching stopper.

By including a region having lower crystallinity than the oxidesemiconductor film 108_2, the oxide semiconductor film 108_1 sometimeshas a high carrier density.

When the oxide semiconductor film 108_1 has a high carrier density, theFermi level is sometimes high relative to the conduction band of theoxide semiconductor film 108_1. This lowers the conduction band minimumof the oxide semiconductor film 108_1, so that the energy differencebetween the conduction band minimum of the oxide semiconductor film108_1 and the trap level, which might be formed in a gate insulatingfilm (here, the insulating film 106), is increased in some cases. Theincrease of the energy difference can reduce trap of charges in the gateinsulating film and reduce variation in the threshold voltage of thetransistor, in some cases. In addition, when the oxide semiconductorfilm 108_1 has a high carrier density, the oxide semiconductor film 108can have high field-effect mobility.

It is favorable to use a composite oxide semiconductor as the oxidesemiconductor film 108_1. The details of the composite oxidesemiconductor will be described in Embodiment 3 or 4.

In the transistor 100 illustrated in FIGS. 5A to 5C, the insulating film106 functions as a gate insulating film of the transistor 100, and theinsulating films 114, 116, and 118 function as protective insulatingfilms of the transistor 100. Furthermore, in the transistor 100, theconductive film 104 functions as a gate electrode, the conductive film112 a functions as a source electrode, and the conductive film 112 bfunctions as a drain electrode. Note that in this specification and thelike, in some cases, the insulating film 106 is referred to as a firstinsulating film, the insulating films 114 and 116 are collectivelyreferred to as a second insulating film, and the insulating film 118 isreferred to as a third insulating film.

<2-2. Components of Semiconductor Device>

Next, components of the semiconductor device in this embodiment aredescribed in detail.

[Substrate]

There is no particular limitation on a material and the like of thesubstrate 102 as long as the material has heat resistance high enough towithstand at least heat treatment to be performed later. For example, aglass substrate, a ceramic substrate, a quartz substrate, a sapphiresubstrate, or the like may be used as the substrate 102. Alternatively,a single crystal semiconductor substrate or a polycrystallinesemiconductor substrate of silicon or silicon carbide, a compoundsemiconductor substrate of silicon germanium, an SOI substrate, or thelike can be used, or any of these substrates provided with asemiconductor element may be used as the substrate 102. In the casewhere a glass substrate is used as the substrate 102, a glass substratehaving any of the following sizes can be used: the 6th generation (1500mm×1850 mm), the 7th generation (1870 mm×2200 mm), the 8th generation(2200 mm×2400 mm), the 9th generation (2400 mm×2800 mm), and the 10thgeneration (2950 mm×3400 mm). Thus, a large-sized display device can befabricated.

Alternatively, a flexible substrate may be used as the substrate 102,and the transistor 100 may be provided directly on the flexiblesubstrate. Alternatively, a separation layer may be provided between thesubstrate 102 and the transistor 100. The separation layer can be usedwhen part or the whole of a semiconductor device formed over theseparation layer is separated from the substrate 102 and transferredonto another substrate. In such a case, the transistor 100 can betransferred to a substrate having low heat resistance or a flexiblesubstrate as well.

[Conductive Film]

The conductive film 104 functioning as a gate electrode and theconductive films 112 a and 112 b functioning as a source electrode and adrain electrode can each be formed using a metal element selected fromchromium (Cr), copper (Cu), aluminum (Al), gold (Au), silver (Ag), zinc(Zn), molybdenum (Mo), tantalum (Ta), titanium (Ti), tungsten (W),manganese (Mn), nickel (Ni), iron (Fe), and cobalt (Co); an alloyincluding any of these metal elements as its component; an alloyincluding a combination of any of these metal elements; or the like.

Furthermore, the conductive films 104, 112 a, and 112 b can be formedusing an oxide conductor or an oxide semiconductor, such as an oxideincluding indium and tin (In—Sn oxide), an oxide including indium andtungsten (In—W oxide), an oxide including indium, tungsten, and zinc(In—W—Zn oxide), an oxide including indium and titanium (In—Ti oxide),an oxide including indium, titanium, and tin (In—Ti—Sn oxide), an oxideincluding indium and zinc (In—Zn oxide), an oxide including indium, tin,and silicon (In—Sn—Si oxide), or an oxide including indium, gallium, andzinc (In—Ga—Zn oxide).

Here, an oxide conductor is described. In this specification and thelike, an oxide conductor may be referred to as OC. For example, oxygenvacancies are formed in an oxide semiconductor, and then hydrogen isadded to the oxygen vacancies, so that a donor level is formed in thevicinity of the conduction band. This increases the conductivity of theoxide semiconductor; accordingly, the oxide semiconductor becomes aconductor. The oxide semiconductor having become a conductor can bereferred to as an oxide conductor. Oxide semiconductors generallytransmit visible light because of their large energy gap. Since an oxideconductor is an oxide semiconductor having a donor level in the vicinityof the conduction band, the influence of absorption due to the donorlevel is small in an oxide conductor, and an oxide conductor has avisible light transmitting property comparable to that of an oxidesemiconductor.

A Cu—X alloy film (X is Mn, Ni, Cr, Fe, Co, Mo, Ta, or Ti) may be usedfor the conductive films 104, 112 a, and 112 b. The use of a Cu—X alloyfilm results in lower manufacturing costs because the film can beprocessed by wet etching.

Among the above-mentioned metal elements, any one or more elementsselected from copper, titanium, tungsten, tantalum, and molybdenum arepreferably included in the conductive films 112 a and 112 b. Inparticular, a tantalum nitride film is preferably used for theconductive films 112 a and 112 b. A tantalum nitride film hasconductivity and a high barrier property against copper or hydrogen.Because a tantalum nitride film releases little hydrogen from itself, itcan be favorably used as the conductive film in contact with the oxidesemiconductor film 108 or the conductive film in the vicinity of theoxide semiconductor film 108. It is favorable to use a copper film forthe conductive films 112 a and 112 b because the resistance of theconductive films 112 a and 112 b can be reduced.

The conductive films 112 a and 112 b can be formed by electrolessplating. As a material that can be deposited by electroless plating, forexample, one or more elements selected from Cu, Ni, Al, Au, Sn, Co, Ag,and Pd can be used. It is further favorable to use Cu or Ag because theresistance of the conductive film can be reduced.

[Insulating Film Functioning as Gate Insulating Film]

As the insulating film 106 functioning as a gate insulating film of thetransistor 100, an insulating layer including at least one of thefollowing films formed by a plasma enhanced chemical vapor deposition(PECVD) method, a sputtering method, or the like can be used: a siliconoxide film, a silicon oxynitride film, a silicon nitride oxide film, asilicon nitride film, an aluminum oxide film, a hafnium oxide film, anyttrium oxide film, a zirconium oxide film, a gallium oxide film, atantalum oxide film, a magnesium oxide film, a lanthanum oxide film, acerium oxide film, and a neodymium oxide film. Note that the insulatingfilm 106 may have a stacked-layer structure or a stacked layer structureof three or more layers.

The insulating film 106 that is in contact with the oxide semiconductorfilm 108 functioning as a channel region of the transistor 100 ispreferably an oxide insulating film and preferably includes a regionincluding oxygen in excess of the stoichiometric composition(oxygen-excess region). In other words, the insulating film 106 is aninsulating film capable of releasing oxygen. In order to provide theoxygen-excess region in the insulating film 106, the insulating film 106is formed in an oxygen atmosphere, or the deposited insulating film 106is subjected to heat treatment in an oxygen atmosphere, for example.

In the case where hafnium oxide is used for the insulating film 106, thefollowing effect is attained. Hafnium oxide has higher dielectricconstant than silicon oxide and silicon oxynitride. Therefore, theinsulating film 106 using hafnium oxide can have a larger thickness thanthe insulating film 106 using silicon oxide, so that leakage current dueto tunnel current can be low. That is, it is possible to provide atransistor with a low off-state current. Moreover, hafnium oxide havinga crystal structure has a higher dielectric constant than hafnium oxidehaving an amorphous structure. Therefore, it is preferable to usehafnium oxide having a crystal structure, in order to provide atransistor with a low off-state current. Examples of the crystalstructure include a monoclinic crystal structure and a cubic crystalstructure. Note that one embodiment of the present invention is notlimited to the above examples.

In this embodiment, a layered film of a silicon nitride film and asilicon oxide film is formed as the insulating film 106. The siliconnitride film has a higher dielectric constant than a silicon oxide filmand needs a larger thickness for capacitance equivalent to that of thesilicon oxide film. Thus, when the silicon nitride film is included inthe gate insulating film of the transistor 100, the thickness of theinsulating film can be increased. This makes it possible to reduce adecrease in withstand voltage of the transistor 100 and furthermore toincrease the withstand voltage, thereby reducing electrostatic dischargedamage to the transistor 100.

[Oxide Semiconductor Film]

The oxide semiconductor film 108 can be formed using the materialsdescribed above.

In the case where the oxide semiconductor film 108 includes In-M-Znoxide, it is preferable that the atomic ratio of metal elements of asputtering target used for forming the In-M-Zn oxide satisfy In>M. Theatomic ratio of metal elements in such a sputtering target is, forexample, In:M:Zn=2:1:3, In:M:Zn=3:1:2, In:M:Zn=4:2:4.1, In:M:Zn=5:1:6,InM:Zn=5:1:7, In:M:Zn=5:1:8, In:M:Zn=6:1:6, In:M:Zn=5:2:5, or the like.

In the case where the oxide semiconductor film 108 is formed of In-M-Znoxide, it is preferable to use a target including polycrystallineIn-M-Zn oxide as the sputtering target. The use of the target includingpolycrystalline In-M-Zn oxide facilitates formation of the oxidesemiconductor film 108 having crystallinity. Note that the atomic ratioof metal elements in the formed oxide semiconductor film 108 varies fromthe above atomic ratios of metal elements of the sputtering targets in arange of ±40%. For example, when a sputtering target with an atomicratio of In to Ga to Zn of 4:2:4.1 is used, the atomic ratio of In to Gato Zn in the formed oxide semiconductor film 108 may be 4:2:3 or in theneighborhood of 4:2:3.

The energy gap of the oxide semiconductor film 108 is 2 eV or more,preferably 2.5 eV or more. With the use of an oxide semiconductor havingsuch a wide energy gap, the off-state current of the transistor 100 canbe reduced.

Furthermore, the oxide semiconductor film 108 preferably has anon-single-crystal structure. Examples of the non-single-crystalstructure include a c-axis-aligned crystalline oxide semiconductor(CAAC-OS) which is described later, a polycrystalline structure, amicrocrystalline structure, and an amorphous structure. Among thenon-single-crystal structures, the amorphous structure has the highestdensity of defect states, whereas the CAAC-OS has the lowest density ofdefect states.

[Insulating Film 1 Functioning as Protective Insulating Film]

The insulating films 114 and 116 function as protective insulating filmsfor the transistor 100. In addition, the insulating films 114 and 116each have a function of supplying oxygen to the oxide semiconductor film108. That is, the insulating films 114 and 116 contain oxygen. Theinsulating film 114 is an insulating film that allows oxygen to passtherethrough. Note that the insulating film 114 also functions as a filmthat relieves damage to the oxide semiconductor film 108 at the time offorming the insulating film 116 in a later step.

A silicon oxide film, a silicon oxynitride film, or the like with athickness greater than or equal to 5 nm and less than or equal to 150nm, preferably greater than or equal to 5 nm and less than or equal to50 nm can be used as the insulating film 114.

In addition, it is preferable that the number of defects in theinsulating film 114 be small and typically, the spin densitycorresponding to a signal that appears at g=2.001 due to a dangling bondof silicon be lower than or equal to 3×10¹⁷ spins/cm³ by electron spinresonance (ESR) measurement. This is because if the density of defectsin the insulating film 114 is high, oxygen is bonded to the defects andthe property of transmitting oxygen of the insulating film 114 islowered.

Note that all oxygen entering the insulating film 114 from the outsidedoes not move to the outside of the insulating film 114 and some oxygenremains in the insulating film 114. Furthermore, movement of oxygenoccurs in the insulating film 114 in some cases in such a manner thatoxygen enters the insulating film 114 and oxygen included in theinsulating film 114 moves to the outside of the insulating film 114.When an oxide insulating film that can transmit oxygen is formed as theinsulating film 114, oxygen released from the insulating film 116provided over the insulating film 114 can be moved to the oxidesemiconductor film 108 through the insulating film 114.

Note that the insulating film 114 can be formed using an oxideinsulating film having a low density of states due to nitrogen oxide.Note that the density of states due to nitrogen oxide can be formedbetween the energy of the valence band maximum (E_(v) _(_) _(os)) andthe energy of the conduction band minimum (E_(c) _(_) _(os)) of theoxide semiconductor film. A silicon oxynitride film that releases lessnitrogen oxide, an aluminum oxynitride film that releases less nitrogenoxide, and the like can be used as the above oxide insulating film.

Note that a silicon oxynitride film that releases less nitrogen oxide isa film which releases ammonia more than nitrogen oxide in thermaldesorption spectroscopy (TDS) analysis; the amount of released ammoniais typically greater than or equal to 1×10¹⁸ cm³ and less than or equalto 5×10¹⁹ cm⁻³. Note that the amount of released ammonia is the amountof ammonia released by heat treatment with which the surface temperatureof a film becomes higher than or equal to 50° C. and lower than or equalto 650° C., preferably higher than or equal to 50° C. and lower than orequal to 550° C.

Nitrogen oxide (NO_(x); x is greater than 0 and less than or equal to 2,preferably greater than or equal to 1 and less than or equal to 2),typically NO₂ or NO, forms levels in the insulating film 114, forexample. The level is positioned in the energy gap of the oxidesemiconductor film 108. Therefore, when nitrogen oxide is diffused tothe interface between the insulating film 114 and the oxidesemiconductor film 108, an electron is in some cases trapped by thelevel on the insulating film 114 side. As a result, the trapped electronremains in the vicinity of the interface between the insulating film 114and the oxide semiconductor film 108; thus, the threshold voltage of thetransistor is shifted in the positive direction.

Nitrogen oxide reacts with ammonia and oxygen in heat treatment. Sincenitrogen oxide included in the insulating film 114 reacts with ammoniaincluded in the insulating film 116 in heat treatment, nitrogen oxideincluded in the insulating film 114 is reduced. Therefore, an electronis hardly trapped at the interface between the insulating film 114 andthe oxide semiconductor film 108.

By using such an oxide insulating film, the insulating film 114 canreduce the shift in the threshold voltage of the transistor, which leadsto a smaller change in the electrical characteristics of the transistor.

Note that in an ESR spectrum at 100 K or lower of the insulating film114, by heat treatment of a manufacturing process of the transistor,typically heat treatment at a temperature higher than or equal to 300°C. and lower than 350° C., a first signal that appears at a g-factor ofgreater than or equal to 2.037 and less than or equal to 2.039, a secondsignal that appears at a g-factor of greater than or equal to 2.001 andless than or equal to 2.003, and a third signal that appears at ag-factor of greater than or equal to 1.964 and less than or equal to1.966 are observed. The split width of the first and second signals andthe split width of the second and third signals that are obtained by ESRmeasurement using an X-band are each approximately 5 mT. The sum of thespin densities of the first signal that appears at a g-factor of greaterthan or equal to 2.037 and less than or equal to 2.039, the secondsignal that appears at a g-factor of greater than or equal to 2.001 andless than or equal to 2.003, and the third signal that appears at ag-factor of greater than or equal to 1.964 and less than or equal to1.966 is lower than 1×10¹⁸ spins/cm³, typically higher than or equal to1×10¹⁷ spins/cm³ and lower than 1×10¹⁸ spins/cm³.

In the ESR spectrum at 100 K or lower, the sum of the spin densities ofthe first signal that appears at a g-factor of greater than or equal to2.037 and less than or equal to 2.039, the second signal that appears ata g-factor of greater than or equal to 2.001 and less than or equal to2.003, and the third signal that appears at a g-factor of greater thanor equal to 1.964 and less than or equal to 1.966 corresponds to the sumof the spin densities of signals attributed to nitrogen oxide (NO_(x); xis greater than 0 and less than or equal to 2, preferably greater thanor equal to 1 and less than or equal to 2). Typical examples of nitrogenoxide include nitrogen monoxide and nitrogen dioxide. In other words,the lower the total spin density of the first signal that appears at ag-factor of greater than or equal to 2.037 and less than or equal to2.039, the second signal that appears at a g-factor of greater than orequal to 2.001 and less than or equal to 2.003, and the third signalthat appears at a g-factor of greater than or equal to 1.964 and lessthan or equal to 1.966 is, the lower the content of nitrogen oxide inthe oxide insulating film is.

The concentration of nitrogen of the above oxide insulating filmmeasured by SIMS is lower than or equal to 6×10²⁰ atoms/cm³.

The above oxide insulating film is formed by a PECVD method at asubstrate temperature higher than or equal to 220° C. and lower than orequal to 350° C. using silane and dinitrogen monoxide, whereby a denseand hard film can be formed.

The insulating film 116 is an oxide insulating film which containsoxygen at a higher proportion than the stoichiometric composition. Partof oxygen is released from the above oxide insulating film by heating.The amount of oxygen released from the oxide insulating film in TDS ismore than or equal to 1.0×10¹⁹ atoms/cm³, preferably more than or equalto 3.0×10²⁰ atoms/cm³. Note that the amount of released oxygen is thetotal amount of oxygen released by heat treatment in a temperature rangeof 50° C. to 650° C. or 50° C. to 550° C. in TDS. In addition, theamount of released oxygen is the total amount of released oxygenconverted into oxygen atoms in TDS.

A silicon oxide film, a silicon oxynitride film, or the like with athickness greater than or equal to 30 nm and less than or equal to 500nm, preferably greater than or equal to 50 nm and less than or equal to400 nm can be used as the insulating film 116.

It is preferable that the number of defects in the insulating film 116be small, and typically the spin density corresponding to a signal thatappears at g=2.001 due to a dangling bond of silicon be lower than1.5×10¹⁸ spins/cm³, preferably lower than or equal to 1×10¹⁸ spins/cm³by ESR measurement. Note that the insulating film 116 is provided moreapart from the oxide semiconductor film 108 than the insulating film 114is; thus, the insulating film 116 may have higher density of defectsthan the insulating film 114.

Furthermore, the insulating films 114 and 116 can be formed usinginsulating films formed of the same kinds of materials; thus, a boundarybetween the insulating films 114 and 116 cannot be clearly observed insome cases. Thus, in this embodiment, the boundary between theinsulating films 114 and 116 is shown by a dashed line. Although atwo-layer structure of the insulating films 114 and 116 is described inthis embodiment, the present invention is not limited to this. Forexample, a single-layer structure of only the insulating film 114 or alayered structure of three or more layers may be employed.

[Insulating Film 2 Functioning as Protective Insulating Film]

The insulating film 118 functions as a protective insulating film forthe transistor 100.

The insulating film 118 includes one or both of hydrogen and nitrogen.Alternatively, the insulating film 118 includes nitrogen and silicon.The insulating film 118 has a function of blocking oxygen, hydrogen,water, alkali metal, alkaline earth metal, or the like. The provision ofthe insulating film 118 makes it possible to prevent outward diffusionof oxygen from the oxide semiconductor film 108, outward diffusion ofoxygen included in the insulating films 114 and 116, and entry ofhydrogen, water, or the like into the oxide semiconductor film 108 fromthe outside.

A nitride insulating film, for example, can be used as the insulatingfilm 118. The nitride insulating film is formed using silicon nitride,silicon nitride oxide, aluminum nitride, aluminum nitride oxide, or thelike.

Although the variety of films such as the conductive films, theinsulating films, the oxide semiconductor film, and the metal film whichare described above can be formed by a sputtering method or a PECVDmethod, such films may be formed by another method, e.g., a thermalchemical vapor deposition (CVD) method. A metal organic chemical vapordeposition (MOCVD) method and an atomic layer deposition (ALD) methodcan be given as examples of a thermal CVD method.

A thermal CVD method has an advantage that no defect due to plasmadamage is generated since it does not utilize plasma for forming a film.In a thermal CVD method, a source gas is introduced into a chamber, thechamber is set at an atmospheric pressure or a reduced pressure, and afilm is deposited on a substrate.

Furthermore, in an ALD method, a source gas is introduced into achamber, the chamber is set at an atmospheric pressure or a reducedpressure, and a film is deposited on a substrate.

<2-3. Structure Example 2 of Semiconductor Device>

Next, variations of the transistor 100 illustrated in in FIGS. 5A to 5Care described with reference to FIGS. 6A to 6C to FIGS. 10A to 10C.

FIG. 6A is a top view of a transistor 100A that is a semiconductordevice of one embodiment of the present invention. FIG. 6B is across-sectional view taken along a dashed dotted line X1-X2 in FIG. 6A.FIG. 6C is a cross-sectional view taken along a dashed dotted line Y1-Y2in FIG. 6A.

Note that the transistor 100A illustrated in FIGS. 6A and 6B is what iscalled a channel-protective transistor. Thus, the semiconductor deviceof one embodiment of the present invention can have either thechannel-etched structure or the channel-protective structure.

In the transistor 100A, the insulating films 114 and 116 have an opening141 a and an opening 141 b. The oxide semiconductor film 108 isconnected to the conductive films 112 a and 112 b through the openings141 a and 141 b. Furthermore, the insulating film 118 is formed over theconductive films 112 a and 112 b. The insulating films 114 and 116function as channel protective films. Note that the other components ofthe transistor 100A are similar to those of the transistor 100 describedabove, and an effect similar to that of the transistor 100 can beobtained.

FIG. 7A is a top view of a transistor 100B that is a semiconductordevice of one embodiment of the present invention. FIG. 7B is across-sectional view taken along a dashed dotted line X1-X2 in FIG. 7A,and FIG. 7C is a cross-sectional view taken along a dashed dotted lineY1-Y2 in FIG. 7A.

The transistor 100B includes the conductive film 104 over the substrate102, the insulating film 106 over the substrate 102 and the conductivefilm 104, the oxide semiconductor film 108 over the insulating film 106,the conductive film 112 a over the oxide semiconductor film 108, theconductive film 112 b over the oxide semiconductor film 108, theinsulating film 114 over the oxide semiconductor film 108, theconductive film 112 a, and the conductive film 112 b, the insulatingfilm 116 over the insulating film 114, a conductive film 120 a over theinsulating film 116, a conductive film 120 b over the insulating film116, and the insulating film 118 over the insulating film 116 and theconductive films 120 a and 120 b.

The insulating films 114 and 116 have an opening 142 a. The insulatingfilms 106, 114, and 116 have an opening 142 b. The conductive film 120 ais electrically connected to the conductive film 104 through the opening142 b. Furthermore, the conductive film 120 b is electrically connectedto the conductive film 112 b through the opening 142 a.

Note that in the transistor 100B, the insulating film 106 functions as afirst gate insulating film of the transistor 100B, the insulating films114 and 116 function as a second gate insulating film of the transistor100B, and the insulating film 118 functions as a protective insulatingfilm of the transistor 100B. In the transistor 100B, the conductive film104 functions as a first gate electrode, the conductive film 112 afunctions as a source electrode, and the conductive film 112 b functionsas a drain electrode. In the transistor 100B, the conductive film 120 afunctions as a second gate electrode, and the conductive film 120 bfunctions as a pixel electrode of a display device.

As illustrated in FIG. 7C, the conductive film 120 a is electricallyconnected to the conductive film 104 through the opening 142 b.Accordingly, the conductive film 104 and the conductive film 120 a aresupplied with the same potential.

As illustrated in FIG. 7C, the oxide semiconductor film 108 ispositioned so as to face the conductive film 104 and the conductive film120 a, and is sandwiched between the two conductive films functioning asthe gate electrodes. The length in the channel length direction and thelength in the channel width direction of the conductive film 120 a arelonger than the length in the channel length direction and the length inthe channel width direction of the oxide semiconductor film 108,respectively. The whole oxide semiconductor film 108 is covered with theconductive film 120 a with the insulating films 114 and 116 positionedtherebetween.

In other words, the conductive film 104 and the conductive film 120 aare connected through the opening provided in the insulating films 106,114, and 116, and each include a region positioned outside an edgeportion of the oxide semiconductor film 108.

With this structure, the oxide semiconductor film 108 included in thetransistor 100B can be electrically surrounded by electric fields of theconductive films 104 and 120 a. A device structure of a transistor, likethat of the transistor 100B, in which electric fields of a first gateelectrode and a second gate electrode electrically surround an oxidesemiconductor film where a channel region is formed can be referred toas a surrounded channel (s-channel) structure.

Since the transistor 100B has the s-channel structure, an electric fieldfor inducing a channel can be effectively applied to the oxidesemiconductor film 108 by the conductive film 104 functioning as a firstgate electrode; therefore, the current drive capability of thetransistor 100B can be improved and high on-state currentcharacteristics can be obtained. Since the on-state current can beincreased, the size of the transistor 100B can be reduced. In addition,since the transistor 100B has a structure in which the oxidesemiconductor film 108 is surrounded by the conductive film 104functioning as the first gate electrode and the conductive film 120 afunctioning as the second gate electrode, the mechanical strength of thetransistor 100B can be increased.

Note that for the conductive films 120 a and 120 b, materials similar tothose described as the materials of the above-described conductive films104, 112 a, and 112 b can be used. In particular, oxide conductor films(OC) are preferable as the conductive films 120 a and 120 b. When theconductive films 120 a and 120 b are formed using an oxide conductivefilm, oxygen can be added to the insulating films 114 and 116.

The other components of the transistor 100B are similar to those of thetransistor 100 described above and have similar effects.

FIG. 8A is a top view of a transistor 100C that is a semiconductordevice of one embodiment of the present invention. FIG. 8B is across-sectional view taken along a dashed dotted line X1-X2 in FIG. 8A,and FIG. 8C is a cross-sectional view taken along a dashed dotted lineY1-Y2 in FIG. 8A.

The transistor 100C is different from the above-described transistor100B in that the conductive films 112 a and 112 b each have athree-layer structure.

The conductive film 112 a of the transistor 100C includes a conductivefilm 112 a_1, a conductive film 112 a_2 over the conductive film 112a_1, and a conductive film 112 a_3 over the conductive film 112 a_2. Theconductive film 112 b of the transistor 100C includes a conductive film112 b_1, a conductive film 112 b_2 over the conductive film 112 b_1, anda conductive film 112 b_3 over the conductive film 112 b_2.

For example, it is preferable that the conductive film 112 a_1, theconductive film 112 b_1, the conductive film 112 a_3, and the conductivefilm 112 b_3 contain one or more elements selected from titanium,tungsten, tantalum, molybdenum, indium, gallium, tin, and zinc.Furthermore, it is preferable that the conductive film 112 a_2 and theconductive film 112 b_2 contain one or more elements selected fromcopper, aluminum, and silver.

Specifically, the conductive film 112 a_1, the conductive film 112 b_1,the conductive film 112 a_3, and the conductive film 112 b_3 can beformed using an In—Sn oxide or an In—Zn oxide and the conductive film112 a 2 and the conductive film 112 b_2 can be formed using copper.

The above structure is preferred because the wiring resistance of theconductive films 112 a and 112 b can be reduced and diffusion of copperto the oxide semiconductor film 108 can be inhibited. The abovestructure is preferred also because the contact resistance between theconductive film 112 b and the conductive film 120 b can be low. Theother components of the transistor 100C are similar to those of thetransistor 100 described above and have similar effects.

FIG. 9A is a top view of a transistor 100D that is a semiconductordevice of one embodiment of the present invention. FIG. 9B is across-sectional view taken along a dashed dotted line X1-X2 in FIG. 9A,and FIG. 9C is a cross-sectional view taken along a dashed dotted lineY1-Y2 in FIG. 9A.

The transistor 100D is different from the above-described transistor100B in that the conductive films 112 a and 112 b each have athree-layer structure. In addition, the transistor 100D is differentfrom the above-described transistor 100C in the shapes of the conductivefilms 112 a and 112 b.

The conductive film 112 a of the transistor 100D includes the conductivefilm 112 a_1, the conductive film 112 a_2 over the conductive film 112a_1, and the conductive film 112 a_3 over the conductive film 112 a 2.The conductive film 112 b of the transistor 100C includes the conductivefilm 112 b_1, the conductive film 112 b_2 over the conductive film 112b_1, and the conductive film 112 b_3 over the conductive film 112 b_2.Note that the conductive film 112 a_1, the conductive film 112 a_2, theconductive film 112 a_3, the conductive film 112 b_1, the conductivefilm 112 b_2, and the conductive film 112 b_3 can be formed using any ofthe above-described materials.

An end portion of the conductive film 112 a_1 has a region locatedoutward from an end portion of the conductive film 112 a_2. Theconductive film 112 a_3 covers a top surface and a side surface of theconductive film 112 a_2 and has a region that is in contact with theconductive film 112 a_1. An end portion of the conductive film 112 b_1has a region located outward from an end portion of the conductive film112 b_2. The conductive film 112 b_3 covers a top surface and a sidesurface of the conductive film 112 b_2 and has a region that is incontact with the conductive film 112 b_1.

The above structure is preferred because the wiring resistance of theconductive films 112 a and 112 b can be reduced and diffusion of copperto the oxide semiconductor film 108 can be inhibited. Note thatdiffusion of copper can be more effectively inhibited in the transistor100D than in the above-described transistor 100C. The above structure ispreferred also because the contact resistance between the conductivefilm 112 b and the conductive film 120 b can be low. The othercomponents of the transistor 100D are similar to those of the transistor100 described above and have similar effects.

FIG. 10A is a top view of a transistor 100E that is a semiconductordevice of one embodiment of the present invention. FIG. 10B is across-sectional view taken along the dashed-dotted line X1-X2 in FIG.10A. FIG. 10C is a cross-sectional view taken along the dashed-dottedline Y1-Y2 in FIG. 10A.

The transistor 100E is different from the transistor 100D describedabove in the position of the conductive films 120 a and 120 b.Specifically, the conductive films 120 a and 120 b of the transistor100E are positioned over the insulating film 118. Note that the othercomponents of the transistor 100E are similar to those of the transistor100D described above, and an effect similar to that of the transistor100D can be obtained.

The structures of the transistors of this embodiment can be freelycombined with each other.

<2-4. Manufacturing Method of Semiconductor Device>

Next, a manufacturing method of the transistor 100B that is asemiconductor device of one embodiment of the present invention isdescribed with reference to FIGS. 11A to 11C, FIGS. 12A to 12C, FIGS.13A to 13C and FIGS. 14A to 14C.

FIGS. 11A to 11C, FIGS. 12A to 12C, FIGS. 13A to 13C and FIGS. 14A to14C are cross-sectional views illustrating a manufacturing method of thesemiconductor device. In each of FIGS. 11A to 11C, FIGS. 12A to 12C,FIGS. 13A to 13C and FIGS. 14A to 14C, the left part is across-sectional view in the channel length direction, and the right partis a cross-sectional view in the channel width direction.

First, a conductive film is formed over the substrate 102 and processedthrough a lithography process and an etching process, whereby theconductive film 104 functioning as the first gate electrode is formed.Then, the insulating film 106 functioning as the first gate insulatingfilm is formed over the conductive film 104 (see FIG. 11A).

In this embodiment, a glass substrate is used as the substrate 102, andas the conductive film 104 functioning as the first gate electrode, a50-nm-thick titanium film and a 200-nm-thick copper film are each formedby a sputtering method. A 400-nm-thick silicon nitride film and a50-nm-thick silicon oxynitride film as the insulating film 106 areformed by a PECVD method.

Note that the above-described silicon nitride film has a three-layerstructure of a first silicon nitride film, a second silicon nitridefilm, and a third silicon nitride film. An example of the three-layerstructure is as follows.

For example, the first silicon nitride film can be formed to have athickness of 50 nm under the conditions where silane at a flow rate of200 sccm, nitrogen at a flow rate of 2000 sccm, and an ammonia gas at aflow rate of 100 sccm are supplied as a source gas to a reaction chamberof a PECVD apparatus, the pressure in the reaction chamber is controlledto 100 Pa, and a power of 2000 W is supplied using a 27.12 MHzhigh-frequency power source.

The second silicon nitride film can be formed to have a thickness of 300nm under the conditions where silane at a flow rate of 200 sccm,nitrogen at a flow rate of 2000 sccm, and an ammonia gas at a flow rateof 2000 sccm are supplied as a source gas to the reaction chamber of thePECVD apparatus, the pressure in the reaction chamber is controlled to100 Pa, and a power of 2000 W is supplied using a 27.12 MHzhigh-frequency power source.

The third silicon nitride film can be formed to have a thickness of 50nm under the conditions where silane at a flow rate of 200 sccm andnitrogen at a flow rate of 5000 sccm are supplied as a source gas to thereaction chamber of the PECVD apparatus, the pressure in the reactionchamber is controlled to 100 Pa, and a power of 2000 W is supplied usinga 27.12 MHz high-frequency power source.

Note that the first silicon nitride film, the second silicon nitridefilm, and the third silicon nitride film can each be formed at asubstrate temperature of lower than or equal to 350° C.

When the silicon nitride film has the above-described three-layerstructure, for example, in the case where a conductive film includingcopper is used as the conductive film 104, the following effect can beobtained.

The first silicon nitride film can inhibit diffusion of copper from theconductive film 104. The second silicon nitride film has a function ofreleasing hydrogen and can improve withstand voltage of the insulatingfilm functioning as a gate insulating film. The third silicon nitridefilm releases a small amount of hydrogen and can inhibit diffusion ofhydrogen released from the second silicon nitride film.

Next, an oxide semiconductor film 108_1_0 and an oxide semiconductorfilm 108_2_0 are formed over the insulating film 106 (see FIGS. 11B and11C).

Note that FIG. 11B is a schematic cross-sectional view illustrating theinside of a deposition apparatus when the oxide semiconductor film108_1_0 and the oxide semiconductor film 108_2_0 are formed over theinsulating film 106. In FIG. 11B, a sputtering apparatus is used as thedeposition apparatus, and a target 191 placed inside the sputteringapparatus and plasma 192 formed under the target 191 are schematicallyillustrated.

Note that in FIG. 11B, oxygen or excess oxygen added to the insulatingfilm 106 is schematically shown by arrows of broken lines. When anoxygen gas is used in forming the oxide semiconductor film 108_1_0,oxygen can be suitably added to the insulating film 106.

First, the oxide semiconductor film 108_1_0 is formed over theinsulating film 106. The thickness of the oxide semiconductor film108_1_0 is greater than or equal to 1 nm and less than or equal to 25nm, preferably greater than or equal to 5 nm and less than or equal to20 nm. The oxide semiconductor film 108_1_0 is formed using one or bothof an inert gas (typically, Ar gas) and an oxygen gas. Note that theproportion of the oxygen gas in the deposition gas for forming the oxidesemiconductor film 108_1_0 (hereinafter also referred to as oxygen flowrate) is higher than or equal to 0% and lower than or equal to 30%,preferably higher than or equal to 5% and lower than or equal to 15%.

When the oxygen flow rate for forming the oxide semiconductor film108_1_0 is set in the above range, the oxide semiconductor film 108_1_0can have lower crystallinity.

Next, the oxide semiconductor film 108_1_0 is heated (not illustrated).Heating of the first oxide semiconductor film in Embodiment 1 may bereferred to for heating of the oxide semiconductor film 108_1_0.

Next, the oxide semiconductor film 108_2_0 is formed over the oxidesemiconductor film 108_1_0. Note that when the oxide semiconductor film108_2_0 is formed, plasma discharge is performed in an atmospherecontaining an oxygen gas. At this time, oxygen is added into the oxidesemiconductor film 108_1_0 over which the oxide semiconductor film108_2_0 is to be formed. The oxygen flow rate in forming the oxidesemiconductor film 108_2_0 is higher than 30% and lower than or equal to100%, preferably higher than or equal to 50% and lower than or equal to100%, further preferably higher than or equal to 70% and lower than orequal to 100%.

The thickness of the oxide semiconductor film 108_2_0 is greater than orequal to 20 nm and less than or equal to 100 nm, preferably greater thanor equal to 20 nm and less than or equal to 50 nm.

Note that the percentage of oxygen flow rate for forming the oxidesemiconductor film 108_2_0 is preferably higher than the percentage ofoxygen flow rate for forming the oxide semiconductor film 108_2_0 asdescribed above. In other words, the oxide semiconductor film 108_1_0 ispreferably formed under a lower oxygen partial pressure than the oxidesemiconductor film 108_2_0.

The substrate temperature in forming the oxide semiconductor film108_1_0 and the oxide semiconductor film 108_2_0 is set at higher thanor equal to room temperature (25° C.) and lower than or equal to 200°C., preferably higher than or equal to room temperature and lower thanor equal to 130° C. Note that it is more favorable to form the oxidesemiconductor film 108_1_0 and the oxide semiconductor film 108_2_0 insuccession in a vacuum because impurities can be prevented from beingcaught at the interfaces.

In addition, increasing the purity of a sputtering gas is necessary. Forexample, as an oxygen gas or an argon gas used as a sputtering gas, agas which is highly purified to have a dew point of −40° C. or lower,preferably −80° C. or lower, further preferably −100° C. or lower, stillfurther preferably −120° C. or lower is used, whereby entry of moistureor the like into the oxide semiconductor film can be minimized.

In the case where the oxide semiconductor film is deposited by asputtering method, a chamber in a sputtering apparatus is preferablyevacuated to be a high vacuum state (to the degree of about 5×10⁻⁷ Pa to1×10⁻⁴ Pa) with an adsorption vacuum evacuation pump such as a cryopumpin order to remove water or the like, which serves as an impurity forthe oxide semiconductor film, as much as possible. In particular, thepartial pressure of gas molecules corresponding to H₂O (gas moleculescorresponding to m/z=18) in the chamber in the standby mode of thesputtering apparatus is preferably lower than or equal to 1×10⁻⁴ Pa,further preferably lower than or equal to 5×10⁻⁵ Pa.

In this embodiment, the oxide semiconductor film 108_1_0 is formed by asputtering method using an In—Ga—Zn oxide target (In:Ga:Zn=4:2:4.1 in anatomic ratio). The substrate temperature during the formation of theoxide semiconductor film 108_1_0 is room temperature, and an argon gasat a flow rate of 180 sccm and an oxygen gas at a flow rate of 20 sccmis used as a deposition gas (percentage of oxygen flow rate: 10%).

The oxide semiconductor film 108_2_0 is formed by a sputtering methodusing an In—Ga—Zn oxide target (In:Ga:Zn=4:2:4.1 in an atomic ratio).The substrate temperature during the formation of the oxidesemiconductor film 108_2_0 is room temperature, and an oxygen gas at aflow rate of 200 sccm is used as a deposition gas (percentage of oxygenflow rate: 100%).

When the percentage of oxygen flow rate in forming the oxidesemiconductor film 108_1_0 is different from that in forming the oxidesemiconductor film 108_2_0, a layered film having a plurality of kindsof crystallinity can be formed.

Next, the oxide semiconductor film 108_1_0 and the oxide semiconductorfilm 108_2_0 are processed into desired shapes, so that theisland-shaped oxide semiconductor film 108_1 and the island-shaped oxidesemiconductor film 108_2 are formed. In this embodiment, the oxidesemiconductor film 108_1 and the oxide semiconductor film 108_2constitute the island-shaped oxide semiconductor film 108 (see FIG.12A).

Heat treatment (hereinafter referred to as first heat treatment) ispreferably performed after the oxide semiconductor film 108 is formed.By the first heat treatment, water, hydrogen, or the like contained inthe oxide semiconductor film 108 can be reduced. The heat treatment forthe purpose of reducing hydrogen, water, and the like may be performedbefore the oxide semiconductor film 108 is processed into an islandshape. Note that the first heat treatment is one kind of treatment forincreasing the purity of the oxide semiconductor film.

The first heat treatment can be performed at a temperature of, forexample, higher than or equal to 150° C. and lower than the strain pointof the substrate, preferably higher than or equal to 200° C. and lowerthan or equal to 450° C., further preferably higher than or equal to250° C. and lower than or equal to 350° C.

Moreover, an electric furnace, an RTA apparatus, or the like can be usedfor the first heat treatment. With the use of an RTA apparatus, the heattreatment can be performed at a temperature higher than or equal to thestrain point of the substrate if the heating time is short. Therefore,the heat treatment time can be shortened. The first heat treatment maybe performed under an atmosphere of nitrogen, oxygen, ultra-dry air (airwith a water content of 20 ppm or less, preferably 1 ppm or less,further preferably 10 ppb or less), or a rare gas (e.g., argon, helium).It is preferable that hydrogen, water, and the like not be contained inthe nitrogen, oxygen, ultra-dry air, or rare gas. Furthermore, afterheat treatment is performed under a nitrogen atmosphere or a rare gasatmosphere, heat treatment may be additionally performed in an oxygenatmosphere or an ultra-dry air atmosphere. As a result, hydrogen, water,and the like can be released from the oxide semiconductor film andoxygen can be supplied to the oxide semiconductor film at the same time.Consequently, the number of oxygen vacancies in the oxide semiconductorfilm can be reduced.

Next, a conductive film 112 is formed over the insulating film 106 andthe oxide semiconductor film 108 (see FIG. 12B).

In this embodiment, as the conductive film 112, a 30-nm-thick titaniumfilm, a 200-nm-thick copper film, and a 10-nm-thick titanium film areformed in this order by a sputtering method.

Next, the conductive film 112 is processed into a desired shape, so thatthe island-shaped conductive film 112 a and the island-shaped conductivefilm 112 b are formed (see FIG. 12C).

In this embodiment, the conductive film 112 is processed with a wetetching apparatus. Note that the method for processing the conductivefilm 112 is not limited to the above-described method, and a dry etchingapparatus may be used, for example.

After the conductive films 112 a and 112 b are formed, a surface (on theback channel side) of the oxide semiconductor film 108 (specifically,the oxide semiconductor film 108_2) may be cleaned. The cleaning may beperformed, for example, using a chemical solution such as a phosphoricacid. The cleaning using a chemical solution such as a phosphoric acidcan remove impurities (e.g., an element included in the conductive films112 a and 112 b) attached to the surface of the oxide semiconductor film108_2. Note that the cleaning is not necessarily performed; in somecases, the cleaning is not performed.

In the step of forming the conductive films 112 a and 112 b and/or thecleaning step, the thickness of a region of the oxide semiconductor film108 which is not covered with the conductive films 112 a and 112 b mightbe reduced.

Note that in the semiconductor device of one embodiment of the presentinvention, the region not covered with the conductive films 112 a and112 b, i.e., an oxide semiconductor film 109_2 is an oxide semiconductorfilm with improved crystallinity. Impurities (in particular, constituentelements used in the conductive films 112 a and 112 b) are not easilydiffused into an oxide semiconductor film with high crystallinity.Accordingly, a highly reliable semiconductor device can be provided.

Although FIG. 12C illustrates an example in which the surface of theoxide semiconductor film 108 not covered with the conductive films 112 aand 112 b, i.e., the surface of the oxide semiconductor film 108_2 has adepression, one embodiment of the present invention is not limited tothis example and the surface of the oxide semiconductor film 108 notcovered with the conductive films 112 a and 112 b does not necessarilyhave a depression.

Next, the insulating film 114 and the insulating film 116 are formedover the oxide semiconductor film 108 and the conductive films 112 a and112 b (see FIG. 13A).

Note that after the insulating film 114 is formed, the insulating film116 is preferably formed successively without exposure to the air. Whenthe insulating film 116 is formed successively after the formation ofthe insulating film 114 without exposure to the air while at least oneof the flow rate of a source gas, the pressure, high-frequency power,and the substrate temperature is adjusted, the concentration ofimpurities attributed to the atmospheric component at the interfacebetween the insulating films 114 and 116 can be reduced.

For example, as the insulating film 114, a silicon oxynitride film canbe formed by a PECVD method. In that case, a deposition gas containingsilicon and an oxidizing gas are preferably used as a source gas.Typical examples of the deposition gas containing silicon includesilane, disilane, trisilane, and silane fluoride.

In this embodiment, a silicon oxynitride film is formed as theinsulating film 114 by a PECVD method under the conditions where thesubstrate 102 is held at a temperature of 220° C., silane at a flow rateof 50 sccm and dinitrogen monoxide at a flow rate of 2000 sccm are usedas a source gas, the pressure in the treatment chamber is 20 Pa, and ahigh-frequency power of 100 W at 13.56 MHz (1.6×10⁻² W/cm² as the powerdensity) is supplied to a parallel-plate electrode.

As the insulating film 116, a silicon oxide film or a silicon oxynitridefilm is formed under the following conditions: the substrate placed inthe treatment chamber of the PECVD apparatus that is vacuum-evacuated isheld at a temperature of higher than or equal to 180° C. and lower thanor equal to 350° C., the pressure in the treatment chamber is higherthan or equal to 100 Pa and lower than or equal to 250 Pa, preferablyhigher than or equal to 100 Pa and lower than or equal to 200 Pa, withintroduction of a source gas into the treatment chamber, and ahigh-frequency power of greater than or equal to 0.17 W/cm² and lessthan or equal to 0.5 W/cm², preferably greater than or equal to 0.25W/cm² and less than or equal to 0.35 W/cm² is supplied to an electrodeprovided in the treatment chamber.

As the deposition conditions of the insulating film 116, thehigh-frequency power having the above power density is supplied to thereaction chamber having the above pressure, whereby the degradationefficiency of the source gas in plasma is increased, oxygen radicals areincreased, and oxidation of the source gas is promoted; thus, the oxygencontent in the insulating film 116 becomes higher than that in thestoichiometric composition. In the film formed at a substratetemperature within the above temperature range, the bond between siliconand oxygen is weak, and accordingly, part of oxygen in the film isreleased by heat treatment in a later step. Thus, it is possible to forman oxide insulating film which contains more oxygen than that in thestoichiometric composition and from which part of oxygen is released byheating.

Note that the insulating film 114 functions as a protective film for theoxide semiconductor film 108 in the step of forming the insulating film116. Therefore, the insulating film 116 can be formed using thehigh-frequency power having a high power density while damage to theoxide semiconductor film 108 is reduced.

Note that in the deposition conditions of the insulating film 116, whenthe flow rate of the deposition gas containing silicon with respect tothe oxidizing gas is increased, the amount of defects in the insulatingfilm 116 can be reduced. Typically, it is possible to form an oxideinsulating film in which the amount of defects is small, i.e., the spindensity of a signal which appears at g=2.001 due to a dangling bond ofsilicon, is lower than 6×10¹⁷ spins/cm³, preferably lower than or equalto 3×10¹⁷ spins/cm³, further preferably lower than or equal to 1.5×10¹⁷spins/cm³ by ESR measurement. As a result, the reliability of thetransistor 100 can be improved.

Heat treatment (hereinafter referred to as second heat treatment) ispreferably performed after the insulating films 114 and 116 are formed.The second heat treatment can reduce nitrogen oxide included in theinsulating films 114 and 116. By the second heat treatment, part ofoxygen contained in the insulating films 114 and 116 can be transferredto the oxide semiconductor film 108, so that the amount of oxygenvacancies included in the oxide semiconductor film 108 can be reduced.

The temperature of the second heat treatment is typically lower than400° C., preferably lower than 375° C., further preferably higher thanor equal to 150° C. and lower than or equal to 350° C. The second heattreatment may be performed in an atmosphere of nitrogen, oxygen,ultra-dry air (air with a water content of less than or equal to 20 ppm,preferably less than or equal to 1 ppm, further preferably less than orequal to 10 ppb), or a rare gas (e.g., argon, helium). It is preferablethat hydrogen, water, and the like not be contained in the nitrogen,oxygen, ultra-dry air, or rare gas. An electric furnace, RTA, or thelike can be used for the heat treatment.

Next, the openings 142 a and 142 b are formed in desired regions in theinsulating films 114 and 116 (see FIG. 13B).

In this embodiment, the openings 142 a and 142 b are formed with a dryetching apparatus. Note that the opening 142 a reaches the conductivefilm 112 b, and the opening 142 b reaches the conductive film 104.

Next, a conductive film 120 is formed over the insulating film 116 (seeFIG. 13C and FIG. 14A).

FIG. 13C is a schematic cross-sectional view illustrating the inside ofa deposition apparatus when the conductive film 120 is formed over theinsulating film 116. In FIG. 13C, a sputtering apparatus is used as thedeposition apparatus, and a target 193 placed inside the sputteringapparatus and plasma 194 formed under the target 193 are schematicallyillustrated.

When the conductive film 120 is formed, plasma discharge is performed inan atmosphere containing an oxygen gas. At this time, oxygen is added tothe insulating film 116 over which the conductive film 120 is to beformed. When the conductive film 120 is formed, an inert gas (e.g., ahelium gas, an argon gas, or a xenon gas) and the oxygen gas may bemixed.

The oxygen gas is mixed at least when the conductive film 120 is formed.The proportion of the oxygen gas in a deposition gas for forming theconductive film 120 is higher than 0% and lower than or equal to 100%,preferably higher than or equal to 10% and lower than or equal to 100%,further preferably higher than or equal to 30% and lower than or equalto 100%.

In FIG. 13C, oxygen or excess oxygen added to the insulating film 116 isschematically shown by arrows of broken lines.

In this embodiment, the conductive film 120 is formed by a sputteringmethod using an In—Ga—Zn oxide target (In:Ga:Zn=4:2:4.1 in an atomicratio).

Note that although oxygen is added to the insulating film 116 when theconductive film 120 is formed in this embodiment, the method for addingoxygen is not limited to this example. For example, oxygen may befurther added to the insulating film 116 after the conductive film 120is formed.

As the method for adding oxygen to the insulating film 116, an ITSO filmwith a thickness of 5 nm may be formed using a target of an oxideincluding indium, tin, and silicon (an In—Sn—Si oxide, also referred toas ITSO) (In₂O₃:SnO₂:SiO₂=85:10:5 in wt %), for example. In that case,the thickness of the ITSO film is preferably greater than or equal to 1nm and less than or equal to 20 nm or greater than or equal to 2 nm andless than or equal to 10 nm, in which case oxygen is favorablytransmitted and release of oxygen can be inhibited. Then, oxygen isadded to the insulating film 116 through the ITSO film. Oxygen can beadded by, for example, ion doping, ion implantation, or plasmatreatment. By application of a bias voltage to the substrate side whenoxygen is added, oxygen can be effectively added to the insulating film116. An ashing apparatus is used, for example, and the power density ofthe bias voltage applied to the substrate side of the ashing apparatuscan be greater than or equal to 1 W/cm² and less than or equal to 5W/cm². The substrate temperature during addition of oxygen is higherthan or equal to room temperature and lower than or equal to 300° C.,preferably higher than or equal to 100° C. and lower than or equal to250° C., whereby oxygen can be added efficiently to the insulating film116.

Next, the conductive film 120 is processed into a desired shape, so thatthe island-shaped conductive films 120 a and 120 b are formed (see FIG.14B).

In this embodiment, the conductive film 120 is processed with a wetetching apparatus.

Next, the insulating film 118 is formed over the insulating film 116 andthe conductive films 120 a and 120 b (see FIG. 14C).

The insulating film 118 includes either or both of hydrogen andnitrogen. As the insulating film 118, a silicon nitride film ispreferably used, for example. The insulating film 118 can be formed by asputtering method or a PECVD method, for example. In the case where theinsulating film 118 is formed by a PECVD method, for example, thesubstrate temperature is lower than 400° C., preferably lower than 375°C., and further preferably higher than or equal to 180° C. and lowerthan or equal to 350° C. The substrate temperature at which theinsulating film 118 is formed is preferably within the above rangebecause a dense film can be formed. Furthermore, when the substratetemperature at which the insulating film 118 is formed is within theabove range, oxygen or excess oxygen in the insulating films 114 and 116can be moved to the oxide semiconductor film 108.

In the case where a silicon nitride film is formed by a PECVD method asthe insulating film 118, a deposition gas containing silicon, nitrogen,and ammonia are preferably used as a source gas. A small amount ofammonia compared with the amount of nitrogen is used, whereby ammonia isdissociated in the plasma and activated species are generated. Theactivated species cleave a bond between silicon and hydrogen which areincluded in a deposition gas including silicon and a triple bond betweennitrogen molecules. As a result, a dense silicon nitride film having fewdefects, in which bonds between silicon and nitrogen are promoted andbonds between silicon and hydrogen are few, can be formed. If the amountof ammonia with respect to nitrogen is large, decomposition of adeposition gas including silicon and decomposition of nitrogen are notpromoted, so that a sparse silicon nitride film in which bonds betweensilicon and hydrogen remain and defects are increased is formed.Therefore, in the source gas, the flow rate of nitrogen is set to bepreferably 5 times or more and 50 times or less, further preferably 10times or more and 50 times or less the flow rate of ammonia.

In this embodiment, with the use of a PECVD apparatus, a 50-nm-thicksilicon nitride film is formed as the insulating film 118 using silane,nitrogen, and ammonia as a source gas. The flow rate of silane is 50sccm, the flow rate of nitrogen is 5000 sccm, and the flow rate ofammonia is 100 sccm. The pressure in the treatment chamber is 100 Pa,the substrate temperature is 350° C., and high-frequency power of 1000 Wis supplied to a parallel-plate electrode with a 27.12 MHzhigh-frequency power source. The PECVD apparatus is a parallel-platePECVD apparatus in which the electrode area is 6000 cm², and the powerper unit area (power density) into which the supplied power is convertedis 1.7×10⁻¹ W/cm².

In the case where the conductive films 120 a and 120 b are formed usingan In—Ga—Zn metal oxide target (In:Ga:Zn=4:2:4.1 in an atomic ratio),either or both of hydrogen and nitrogen included in the insulating film118 might enter the conductive films 120 a and 120 b. In this case,either or both of hydrogen and nitrogen might be bonded to oxygenvacancies in the conductive films 120 a and 120 b to cause a reductionin the resistance of the conductive films 120 a and 120 b.

After the insulating film 118 is formed, heat treatment similar to thefirst heat treatment or the second heat treatment (hereinafter referredto as third heat treatment) may be performed.

By the third heat treatment, oxygen included in the insulating film 116moves into the oxide semiconductor film 108 to fill the oxygen vacanciesin the oxide semiconductor film 108.

Through the above process, the transistor 100B illustrated in FIGS. 7Ato 7C can be manufactured.

The transistor 100 illustrated in FIGS. 5A to 5C can be manufactured byforming the insulating film 118 after the step of FIG. 13A. Thetransistor 100A illustrated in FIGS. 6A to 6C can be manufactured bychanging the formation order of the conductive films 112 a and 112 b andthe insulating films 114 and 116 and, in addition, adding a step forforming the openings 141 a and 141 b in the insulating films 114 and116.

At least part of this embodiment can be implemented in combination withany of the other embodiments and the other examples described in thisspecification as appropriate.

Embodiment 3

In this embodiment, the composition of a cloud-aligned composite oxidesemiconductor (CAC-OS) included in the oxide semiconductor film of oneembodiment of the present invention is described.

<3-1. Composition of CAC>

The CAC has, for example, a composition in which elements included in anoxide semiconductor film are unevenly distributed. Materials includingunevenly distributed elements each have a size of greater than or equalto 0.5 nm and less than or equal to 10 nm, preferably greater than orequal to 1 nm and less than or equal to 2 nm, or a similar size. Notethat in the following description of an oxide semiconductor film, astate in which one or more metal elements are mixed is referred to as amosaic pattern or a patch-like pattern. Regions of the metal elementeach have a size of greater than or equal to 0.5 nm and less than orequal to 10 nm, preferably greater than or equal to 1 nm and less thanor equal to 2 nm, or a similar size.

For example, of In—Ga—Zn oxide (hereinafter also referred to as IGZO),CAC-IGZO has a composition in which materials are separated into indiumoxide (InO_(X1), where X1 is a real number greater than 0) or indiumzinc oxide (In_(X2)Zn_(Y2)O_(Z2), where X2, Y2, and Z2 are real numbersgreater than 0), and gallium oxide (GaO_(X3), where X3 is a real numbergreater than 0), gallium zinc oxide (Ga_(X4)Zn_(Y4)O_(Z4), where X4, Y4,and Z4 are real numbers greater than 0), or the like, and a mosaicpattern is formed. Then, InO_(X1) or In_(X2)Zn_(Y2)O_(Z2) forming themosaic pattern is distributed in the oxide semiconductor film. Thiscomposition is also referred to as a cloud-like composition.

That is, CAC-IGZO is a composite oxide semiconductor film having acomposition in which a region including In_(X2)Zn_(Y2)O_(Z2) or InO_(X1)as a main component and a region including GaO_(X3) as a main componentare distributed unevenly and mixed. Surrounding portions of the regionincluding In_(X2)Zn_(Y2)O_(Z2) or InO_(X1) as a main component and theregion including GaO_(X3) as a main component are unclear (blurred), sothat a boundary is not clearly observed in some cases.

Note that a compound including In, Ga, Zn, and O is also known as IGZO.Typical examples of IGZO include a crystalline compound represented byInGaO₃(ZnO)_(m1) (m1 is a natural number) and a crystalline compoundrepresented by In_((1+x0))Ga_((1−x0))O₃(ZnO)_(m0) (−1≤x0≤1; m0 is agiven number).

The above crystalline compounds have a single crystal structure, apolycrystalline structure, or a CAAC structure. Note that the CAACstructure is a crystal structure in which a plurality of IGZOnanocrystals have c-axis alignment and are connected in the a-b planedirection without alignment.

On the other hand, the CAC relates to the material composition. In amaterial composition of a CAC including In, Ga, Zn, and O, nanoparticleregions including Ga as a main component are observed in part of the CACand nanoparticle regions including In as a main component are observedin part thereof. Therefore, the crystal structure is a secondary elementfor the CAC composition. Note that the region including Ga as a maincomponent and the region including In as a main component can beexamined by EDX mapping. Note that the region including Ga as a maincomponent and the region including In as a main component may each bereferred to as a nanoparticle. The diameter of the nanoparticle isgreater than or equal to 0.5 nm and less than or equal to 10 nm,typically greater than or equal to 1 nm and less than or equal to 2 nm.Surrounding portions of the nanoparticles are unclear (blurred), so thata boundary is not clearly observed in some cases.

Note that in the CAC composition, a stacked-layer structure includingtwo or more films with different atomic ratios is not included. Forexample, a two-layer structure of a film including In as a maincomponent and a film including Ga as a main component is not included.

A boundary between the region including GaO_(X3) as a main component andthe region including In_(X2)Zn_(Y2)O_(Z2) or InO_(X1) as a maincomponent is not clearly observed in some cases. For example, thedensity of an element which is a main component is gradually loweredfrom the central portion of the region toward the surrounding portion.For example, the number (abundance) of atoms of an element countable inan EDX mapping image of a cross-sectional photograph has a gradualchange, and thus, the surrounding portion of the region is unclear(blurred) in the EDX mapping image. Specifically, from the centralportion toward the surrounding portion in the region including GaO_(X3)as a main component, the number of Ga atoms gradually reduces and thenumber of Zn atoms gradually increases, so that a region includingGa_(X)Zn_(Y)O_(Z) as a main component gradually appears. Accordingly,the surrounding portion of the region including GaO_(X3) as a maincomponent is unclear (blurred) in the EDX mapping image.

<3-2. Analysis of CAC-IGZO>

Next, measurement results of an oxide semiconductor film over asubstrate by a variety of methods are described.

[Structure of Samples and Formation Method Thereof]

Nine samples of one embodiment of the present invention are describedbelow. The samples are formed at different substrate temperatures andwith different ratios of an oxygen gas flow rate in formation of theoxide semiconductor film. Note that each sample includes a substrate andan oxide semiconductor film over the substrate.

A method for forming the samples is described.

A glass substrate is used as the substrate. Over the glass substrate, a100-nm-thick In—Ga—Zn oxide is formed as an oxide semiconductor filmwith a sputtering apparatus. The formation conditions are as follows:the pressure in a chamber is 0.6 Pa, and a metal oxide target (with anatomic ratio of In:Ga:Zn=4:2:4.1) is used as a target. The metal oxidetarget provided in the sputtering apparatus is supplied with an AC powerof 2500 W.

As for the conditions in the formation of the oxide of the nine samples,the substrate temperature is set to a room temperature (hereinafter sucha temperature is also referred to as R.T.), to 130° C., and to 170° C.The ratio of a flow rate of an oxygen gas to a flow rate of a mixed gasof Ar and oxygen (also referred to as an oxygen gas flow rate ratio) isset to 10%, 30%, and 100%.

[Analysis by X-Ray Diffraction]

In this section, results of X-ray diffraction (XRD) measurementperformed on the nine samples are described. As an XRD apparatus, D8ADVANCE manufactured by Bruker AXS is used. The conditions are asfollows: scanning is performed by an out-of-plane method at θ/2θ, thescanning range is 15 deg. to 50 deg., the step width is 0.02 deg., andthe scanning speed is 3.0 deg./min.

FIG. 16 shows an XRD spectra measured by an out-of-plane method. In FIG.16, the top row shows the measurement results of the samples formed at asubstrate temperature of 170° C.; the middle row shows the measurementresults of the samples formed at a substrate temperature of 130° C.; thebottom row shows the measurement results of the samples formed at asubstrate temperature of R.T. The left column shows the measurementresults of the samples formed with an oxygen gas flow rate ratio of 10%;the middle column shows the measurement results of the samples formedwith an oxygen gas flow rate ratio of 30%; the right column shows themeasurement results of the samples formed with an oxygen gas flow rateratio of 100%.

In the XRD spectra shown in FIG. 16, the higher the substratetemperature at the time of formation is or the higher the oxygen gasflow rate ratio at the time of formation is, the higher the intensity ofthe peak at around 2θ=31° is. Note that it is found that the peak ataround 2θ=31° is derived from a crystalline IGZO compound whose c-axesare aligned in a direction substantially perpendicular to a formationsurface or a top surface of the crystalline IGZO compound (such acompound is also referred to as c-axis aligned crystalline (CAAC) IGZO).

As shown in the XRD spectra in FIG. 16, as the substrate temperature atthe time of formation is lower or the oxygen gas flow rate ratio at thetime of formation is lower, a peak becomes less clear. Accordingly, itis found that there are no alignment in the a-b plane direction andc-axis alignment in the measured areas of the samples that are formed ata lower substrate temperature or with a lower oxygen gas flow rateratio.

[Analysis with Electron Microscope]

This section describes the observation and analysis results of thesamples formed at a substrate temperature of R.T. and with an oxygen gasflow rate ratio of 10% with a high-angle annular dark-field scanningtransmission electron microscope (HAADF-STEM). An image obtained with anHAADF-STEM is also referred to as a TEM image.

Described are the results of image analysis of plan-view images andcross-sectional images obtained with an HAADF-STEM (also referred to asplan-view TEM images and cross-sectional TEM images, respectively). TheTEM images are observed with a spherical aberration corrector function.The HAADF-STEM images are obtained using an atomic resolution analyticalelectron microscope JEM-ARM200F manufactured by JEOL Ltd. under thefollowing conditions: the acceleration voltage is 200 kV, andirradiation with an electron beam with a diameter of approximately 0.1nm is performed.

FIG. 17A is a plan-view TEM image of the sample formed at a substratetemperature of R.T. and an oxygen gas flow rate ratio of 10%. FIG. 17Bis a cross-sectional TEM image of the sample formed at a substratetemperature of R.T. and with an oxygen gas flow rate ratio of 10%.

[Analysis of Electron Diffraction Patterns]

This section describes electron diffraction patterns obtained byirradiation of the sample formed at a substrate temperature of R.T. andan oxygen gas flow rate ratio of 10% with an electron beam with a probediameter of 1 nm (also referred to as a nanobeam).

Electron diffraction patterns of points indicated by black dots a1, a2,a3, a4, and a5 in the plan-view TEM image in FIG. 17A of the sampleformed at a substrate temperature of R.T. and an oxygen gas flow rateratio of 10% are observed. Note that the electron diffraction patternsare observed while electron beam irradiation is performed at a constantrate for 35 seconds. FIGS. 17C, 17D, 17E, 17F, and 17G show the resultsof the points indicated by the black dots a1, a2, a3, a4, and a5,respectively.

In FIGS. 17C, 17D, 17E, 17F, and 17G, regions with high luminance in acircular (ring) pattern can be shown. Furthermore, a plurality of spotscan be shown in a ring-like shape.

Electron diffraction patterns of points indicated by black dots b1, b2,b3, b4, and b5 in the cross-sectional TEM image in FIG. 17B of thesample formed at a substrate temperature of R.T. and an oxygen gas flowrate ratio of 10% are observed. FIGS. 17H, 17I, 17J, 17K, and 17L showthe results of the points indicated by the black dots b1, b2, b3, b4,and b5, respectively.

In FIGS. 17H, 17I, 17J, 17K, and 17L, regions with high luminance in aring pattern can be shown. Furthermore, a plurality of spots can beshown in a ring-like shape.

For example, when an electron beam with a probe diameter of 300 nm isincident on a CAAC-OS including an InGaZnO₄ crystal in a directionparallel to the sample surface, a diffraction pattern including a spotderived from the (009) plane of the InGaZnO₄ crystal is obtained. Thatis, the CAAC-OS has c-axis alignment and the c-axes are aligned in thedirection substantially perpendicular to the formation surface or thetop surface of the CAAC-OS. Meanwhile, a ring-like diffraction patternis shown when an electron beam with a probe diameter of 300 nm isincident on the same sample in a direction perpendicular to the samplesurface. That is, it is found that the CAAC-OS has neither a-axisalignment nor b-axis alignment.

Furthermore, a diffraction pattern like a halo pattern is observed whenan oxide semiconductor film including a nanocrystal (a nanocrystallineoxide semiconductor (nc-OS)) is subjected to electron diffraction usingan electron beam with a large probe diameter (e.g., 50 nm or larger).Meanwhile, bright spots are shown in a nanobeam electron diffractionpattern of the nc-OS obtained using an electron beam with a small probediameter (e.g., smaller than 50 nm). Furthermore, in a nanobeam electrondiffraction pattern of the nc-OS, regions with high luminance in acircular (ring) pattern are shown in some cases. Also in a nanobeamelectron diffraction pattern of the nc-OS, a plurality of bright spotsare shown in a ring-like shape in some cases.

The electron diffraction pattern of the sample formed at a substratetemperature of R.T. and with an oxygen gas flow rate ratio of 10% hasregions with high luminance in a ring pattern and a plurality of brightspots appear in the ring-like pattern. Accordingly, the sample formed ata substrate temperature of R.T. and with an oxygen gas flow rate ratioof 10% exhibits an electron diffraction pattern similar to that of thenc-OS and does not show alignment in the plane direction and thecross-sectional direction.

According to what is described above, an oxide semiconductor film formedat a low substrate temperature or with a low oxygen gas flow rate ratiois likely to have characteristics distinctly different from those of anoxide semiconductor film having an amorphous structure and an oxidesemiconductor film having a single crystal structure.

[Elementary Analysis]

This section describes the analysis results of elements included in thesample formed at a substrate temperature of R.T. and with an oxygen gasflow rate ratio of 10%. For the analysis, by energy dispersive X-rayspectroscopy (EDX), EDX mapping images are obtained. An energydispersive X-ray spectrometer AnalysisStation JED-2300T manufactured byJEOL Ltd. is used as an elementary analysis apparatus in the EDXmeasurement. A Si drift detector is used to detect an X-ray emitted fromthe sample.

In the EDX measurement, an EDX spectrum of a point is obtained in such amanner that electron beam irradiation is performed on the point in adetection target region of a sample, and the energy of characteristicX-ray of the sample generated by the irradiation and its frequency aremeasured. In this example, peaks of an EDX spectrum of the point areattributed to electron transition to the L shell in an In atom, electrontransition to the K shell in a Ga atom, and electron transition to the Kshell in a Zn atom and the K shell in an O atom, and the proportions ofthe atoms in the point are calculated. An EDX mapping image indicatingdistributions of proportions of atoms can be obtained through theprocess in an analysis target region of a sample.

FIGS. 18A to 18C show EDX mapping images in a cross section of thesample formed at a substrate temperature of R.T. and with an oxygen gasflow rate ratio of 10%. FIG. 18A shows an EDX mapping image of Ga atoms.The proportion of the Ga atoms in all the atoms is 1.18 atomic % to18.64 atomic %. FIG. 18B shows an EDX mapping image of In atoms. Theproportion of the In atoms in all the atoms is 9.28 atomic % to 33.74atomic %. FIG. 18C shows an EDX mapping image of Zn atoms. Theproportion of the Zn atoms in all the atoms is 6.69 atomic % to 24.99atomic %. FIGS. 18A to 18C show the same region in the cross section ofthe sample formed at a substrate temperature of R.T. and with an oxygengas flow rate ratio of 10%. In the EDX mapping images, the proportion ofan element is indicated by grayscale: the more measured atoms exist in aregion, the brighter the region is; the less measured atoms exist in aregion, the darker the region is. The magnification of the EDX mappingimages in FIGS. 18A to 18C is 7200000 times.

The EDX mapping images in FIGS. 18A to 18C show relative distribution ofbrightness indicating that each element has a distribution in the sampleformed at a substrate temperature of R.T. and with an oxygen gas flowrate ratio of 10%. Areas surrounded by solid lines and areas surroundedby dashed lines in FIGS. 18A to 18C are examined.

In FIG. 18A, a relatively dark region occupies a large area in the areasurrounded by the solid line, while a relatively bright region occupiesa large area in the area surrounded by the dashed line. In FIG. 18B, arelatively bright region occupies a large area in the area surrounded bythe solid line, while a relatively dark region occupies a large area inthe area surrounded by the dashed line.

That is, the areas surrounded by the solid lines are regions including arelatively large number of In atoms and the areas surrounded by thedashed lines are regions including a relatively small number of Inatoms. In FIG. 18C, the right portion of the area surrounded by thesolid line is relatively bright and the left portion thereof isrelatively dark. Thus, the area surrounded by the solid line is a regionincluding In_(X2)Zn_(Y2)O_(Z2), InO_(X1), or the like as a maincomponent.

The area surrounded by the solid line is a region including a relativelysmall number of Ga atoms and the area surrounded by the dashed line is aregion including a relatively large number of Ga atoms. In FIG. 18C, theupper left portion of the area surrounded by the dashed line isrelatively bright and the lower right portion thereof is relativelydark. Thus, the area surrounded by the dashed line is a region includingGaO_(X3), Ga_(X4)Zn_(Y4)O_(Z4), or the like as a main component.

Furthermore, as shown in FIGS. 18A to 18C, the In atoms are relativelymore uniformly distributed than the Ga atoms, and regions includingInO_(X1) as a main component are seemingly joined to each other througha region including In_(X2)Zn_(Y2)O_(Z2) as a main component. Thus, theregions including In_(X2)Zn_(Y2)O_(Z2) and InO_(X1) as main componentsextend like a cloud.

An In—Ga—Zn oxide having a composition in which the regions includingGaO as a main component and the regions including In_(X2)Zn_(Y2)O_(Z2)or InO_(X1) as main components are unevenly distributed and mixed can bereferred to as CAC-IGZO.

As shown in FIGS. 18A to 18C, each of the regions including GaO_(X3) orthe like as a main component and the regions includingIn_(X2)Zn_(Y2)O_(Z2) or InO_(X1) as a main component has a size ofgreater than or equal to 0.5 nm and less than or equal to 10 nm, orgreater than or equal to 0.3 nm and less than or equal to 3 nm. Notethat it is preferable that a diameter of a region including each metalelement as a main component be greater than or equal to 1 nm and lessthan or equal to 2 nm in the EDX mapping images.

As described above, CAC-IGZO has a structure different from that of anIGZO compound in which metal elements are evenly distributed, and hascharacteristics different from those of the IGZO compound. That is,CAC-IGZO includes regions including GaO_(X3) or the like as a maincomponent and regions including In_(X2)Zn_(Y2)O_(Z2) or InO_(X1) as amain component. Accordingly, when CAC-IGZO is used for a semiconductorelement, the property derived from GaO_(X3) or the like and the propertyderived from In_(X2)Zn_(Y2)O_(Z2) or InO_(X1) complement each other,whereby high on-state current (Ion) and high field-effect mobility (μ)can be achieved.

Note that the conduction mechanism of a semiconductor element includingCAC-IGZO that achieves high on-state current (Ion) and high field-effectmobility (μ) can be presumed by a random-resistance-network model inpercolation theory.

A semiconductor element including CAC-IGZO has high reliability. Thus,CAC-IGZO is suitably used in a variety of semiconductor devices typifiedby a display.

At least part of this embodiment can be implemented in combination withany of the other embodiments described in this specification asappropriate.

Embodiment 4

In this embodiment, an oxide semiconductor film of one embodiment of thepresent invention is described with reference to FIG. 19A to FIG. 24C.

<4-1. Oxide Semiconductor Film>

The oxide semiconductor film preferably contains at least indium. Inparticular, indium and zinc are preferably contained. In addition,gallium, aluminum, yttrium, tin, or the like is preferably contained.One or more elements selected from boron, silicon, titanium, iron,nickel, germanium, zirconium, molybdenum, lanthanum, cerium, neodymium,hafnium, tantalum, tungsten, magnesium, or the like may be contained.

Here, the oxide semiconductor film is assumed to contain indium, anelement M, and zinc. Note that the element M is gallium, aluminum,yttrium, tin, or the like. Other elements that can be used as theelement M include boron, silicon, titanium, iron, nickel, germanium,zirconium, molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum,tungsten, and magnesium. Note that two or more of the above elements maybe used in combination as the element M. In the description below, theterms of the atomic ratio of indium to the element M and zinc containedin the oxide semiconductor film are denoted by [In], [M], and [Zn],respectively, in some cases.

<4-2. Crystal Structure of Oxide Semiconductor Film>

An oxide semiconductor is classified into a single crystal oxidesemiconductor and a non-single-crystal oxide semiconductor. Examples ofa non-single-crystal oxide semiconductor include a c-axis-alignedcrystalline oxide semiconductor (CAAC-OS), a polycrystalline oxidesemiconductor, a nanocrystalline oxide semiconductor (nc-OS), anamorphous-like oxide semiconductor (a-like OS), and an amorphous oxidesemiconductor.

The CAAC-OS has c-axis alignment, its nanocrystals are connected in thea-b plane direction, and its crystal structure has distortion. Note thatthe distortion in the CAAC-OS is a portion where the direction of alattice arrangement changes between a region with a regular latticearrangement and another region with a regular lattice arrangement in aregion in which nanocrystals are connected.

The shape of the nanocrystal is basically hexagon; however, the shape isnot always a regular hexagon and is a non-regular hexagon in some cases.At the distortion, a polygonal nanocrystal such as a pentagonalnanocrystal or a heptagonal nanocrystal is included in some cases. Notethat a clear grain boundary cannot be observed even in the vicinity ofdistortion in the CAAC-OS. That is, a lattice arrangement is distortedso that formation of a grain boundary is inhibited. This is probablybecause the CAAC-OS can tolerate distortion owing to a low density ofthe atomic arrangement in an a-b plane direction, the interatomic bonddistance changed by substitution of a metal element, and the like.

The CAAC-OS tends to have a layered crystal structure (also referred toas a layered structure) in which a layer containing indium and oxygen(hereinafter, In layer) and a layer containing the element M, zinc, andoxygen (hereinafter, (M,Zn) layer) are stacked. Note that indium and theelement M can be replaced with each other, and when the element M of the(M,Zn) layer is replaced with indium, the layer can also be referred toas an (In,M,Zn) layer. When indium of the In layer is replaced with theelement M, the layer can also be referred to as an (In,M) layer.

In the nc-OS, a microscopic region (for example, a region with a sizegreater than or equal to 0.5 nm and less than or equal to 10 nm, inparticular, a region with a size greater than or equal to 1 nm and lessthan or equal to 2 nm) has a periodic atomic arrangement. There is noregularity of crystal orientation between different nanocrystals in thenc-OS. Thus, the orientation of the whole film is not observed.Accordingly, the nc-OS cannot be distinguished from an a-like OS or anamorphous oxide semiconductor, depending on an analysis method.

The a-like OS has a structure between those of the nc-OS and theamorphous oxide semiconductor. The a-like OS contains a void or alow-density region. That is, the a-like OS has an unstable structure ascompared with the nc-OS and the CAAC-OS.

An oxide semiconductor can have various structures which show variousdifferent properties. Two or more of the amorphous oxide semiconductor,the polycrystalline oxide semiconductor, the a-like OS, the nc-OS, andthe CAAC-OS may be included in an oxide semiconductor of one embodimentof the present invention.

The oxide semiconductor film of one embodiment of the present inventionincludes a composite oxide semiconductor. Therefore, an oxidesemiconductor film may be referred to as a composite oxide semiconductorin the description below. The use of a composite oxide semiconductor canprovide a transistor having high field effect mobility.

<4-3. Composite Oxide Semiconductor>

A composite oxide semiconductor in one embodiment of the presentinvention is described. Samples including oxide semiconductor films areformed, and the results of evaluating the samples are described below.

[Structure of Samples and Formation Method Thereof]

Two samples of one embodiment of the present invention are formed andevaluated. The samples are formed at different substrate temperaturesand with different oxygen gas flow rate ratios at the time of formingthe oxide semiconductor films. Note that in the following description,the two samples are referred to as Sample A1 and Sample A2 forconvenience.

Sample A1 and Sample A2 each include a substrate and an oxidesemiconductor film over the substrate.

[Sample A1]

A method for forming Sample A1 is described. In this case, a glasssubstrate is used as the substrate. Over the glass substrate, a100-nm-thick In—Ga—Zn oxide is formed as an oxide semiconductor filmwith a sputtering apparatus. The formation conditions are as follows:the pressure in a chamber is 0.6 Pa, and a metal oxide target (with anatomic ratio of In:Ga:Zn=4:2:4.1) is used as a target. The metal oxidetarget provided in the sputtering apparatus is supplied with an AC powerof 2500 W. As for the conditions in the formation of the oxide, thesubstrate temperature is set to R.T. As a mixed gas, an Ar gas and anoxygen gas are supplied at 270 sccm and 30 sccm, respectively, into thedeposition chamber to form the oxide. That is, the ratio of the flowrate of an oxygen gas to a flow rate of a mixed gas of Ar and oxygen(also referred to as an oxygen gas flow rate ratio) is set to 10%.

[Sample A2]

A method for forming Sample A2 is described. In this case, a glasssubstrate is used as the substrate. Over the glass substrate, a100-nm-thick In—Ga—Zn oxide is formed as an oxide semiconductor filmwith a sputtering apparatus. The formation conditions are as follows:the pressure in a chamber is 0.6 Pa, and a metal oxide target (with anatomic ratio of In:Ga:Zn=4:2:4.1) is used as a target. The metal oxidetarget provided in the sputtering apparatus is supplied with an AC powerof 2500 W. Note that the substrate temperature at the time of formingthe oxide is set to 170° C. An oxygen gas is supplied at 300 sccm intothe deposition chamber to form the oxide. That is, the ratio of the flowrate of an oxygen gas to a flow rate of a whole gas at the time offorming the oxide (also referred to as an oxygen gas flow rate ratio) isset to 100%.

[Cross-Sectional HAADF-STEM Observation]

Next, the cross-sectional observation of Samples A1 and A2 formed asdescribed above is described. As the cross-sectional observation,HAADF-STEM observation was performed. Note that for the HAADF-STEMobservation, JEM-ARM200F manufactured by JEOL Ltd. was used under theconditions where the acceleration voltage was 200 kV. FIGS. 19A and 19Bare HAADF-STEM images of Sample A1 and Sample A2, respectively.

In the HAADF-STEM image of Sample A1 shown in FIG. 19A, no clearalignment is observed. In contrast, in the HAADF-STEM image of Sample A2shown in FIG. 19B, layered alignment in the c-axis direction isobserved.

FIG. 20 is a schematic view of the cross section of Sample A1 shown inFIG. 19A.

In FIG. 20, a schematic view of a cross section (here, referred to as ac-axis direction) of the oxide semiconductor film formed over asubstrate Sub. is shown. FIG. 20 illustrates an example in which theoxide semiconductor film is formed over the substrate; however, oneembodiment of the present invention is not limited to this example andan insulating film such as a base film or an interlayer film or anothersemiconductor film such as an oxide semiconductor film may be formedbetween the substrate and the oxide semiconductor film.

The oxide semiconductor film of one embodiment of the present inventionis a composite oxide semiconductor having a structure in which RegionA1, Region B1, and Region C1 are mixed as illustrated in FIG. 20.

Region A1 shown in FIG. 20 is a region including In with[In]:[M]:[Zn]=x:y:z (x>0, y≥0, z≥0). Region B1 is a region including Gawith [In]:[M]:[Zn]=a:b:c (a≥0, b>0, c≥0). Region C1 is a regionincluding Zn with [In]:[M]:[Zn]=α:β:γ (α≥0, β≥0, γ>0).

Note that in this specification, when the atomic ratio of In to theelement M in Region A1 is greater than the atomic ratio of In to theelement M in Region B1, Region A1 has a higher In concentration thanRegion B1. Therefore, in this specification, Region A1 is also referredto as an In-rich region, and Region B1 is also referred to as an In-poorregion.

For example, the In concentration in Region A1 is 1.1 or more times,preferably 2 to 10 times that in Region B1. The region A1 is an oxidecontaining at least In and does not necessarily contain the element Mand Zn.

In the oxide semiconductor film of one embodiment of the presentinvention, Region A1, Region B1, and Region C1 form a composite. Thatis, carrier movement occurs easily in Region A1, whereas carriermovement does not occur easily in Region B1. Therefore, the oxidesemiconductor of one embodiment of the present invention can be used asa material with high carrier mobility, excellent switchingcharacteristics, and favorable semiconductor characteristics. Note thatRegion C1 is indium zinc oxide, gallium zinc oxide, or indium galliumzinc oxide. Therefore, Region C1 can contribute to both carrier mobilityand switching characteristics.

In other words, Region A1 has a lower semiconductor property and higherconductivity than Region B1. Conversely, Region B1 has a highersemiconductor property and lower conductivity than Region A1. Here, ahigh semiconductor property means a wide band gap, favorable switchingcharacteristics, being close to an i-type semiconductor, or the like.

For example, as shown in FIG. 20, a plurality of Regions A1 are presentin particulate form (in cluster form) in the c-axis direction. Note thatclusters may be distributed unevenly and irregularly. A plurality ofclusters overlap each other or are connected to each other in somecases. For example, in some cases, shapes each including a clusteroverlapping with another cluster are connected to each other, so thatRegion A1 is observed to extend in a cloud-like manner.

In other words, the clusters contained in Region A1 (also referred to asfirst clusters) have a lower semiconductor property and higherconductivity than the clusters contained in Region B1 (also referred toas second clusters). Conversely, the clusters contained in Region B1have a higher semiconductor property and lower conductivity than theclusters contained in Region A1. In this structure, Region B1 includes aplurality of second clusters and has a portion where the plurality ofsecond clusters is connected to each other. In other words, theplurality of first clusters included in Region A1 includes a portionwhere the first cluster and another first cluster are connected to eachother in a cloud-like manner, and the plurality of second clustersincluded in Region B1 includes a portion where the second cluster andanother second cluster are connected to each other in a cloud-likemanner.

In other words, in the composite oxide semiconductor of one embodimentof the present invention, a first region (Region A1) with a high Inconcentration and a second region (Region B1) with a low Inconcentration are connected in a cloud-like manner through Region C1.Alternatively, in the composite oxide semiconductor of one embodiment ofthe present invention, the first region where In is distributed at ahigh concentration and the second region where In is not distributed ata high concentration are connected in a cloud-like manner.

As shown in FIG. 20, Regions A1 are connected to each other, so thatRegions A1 can serve as a current path. Accordingly, the oxidesemiconductor film can have increased conductivity and a transistorusing this oxide semiconductor film can have increased field-effectmobility.

In other words, Regions B1 shown in FIG. 20 are scattered in Regions A1.Therefore, Region B1 can exist in a state of being sandwichedthree-dimensionally by Regions A1. In other words, Region B1 can existin a state of being surrounded by Region A1. That is, Region B1 isenclosed by Region A1.

Note that the proportion of scattered Regions A1 can be adjusted bychanging, for example, the formation conditions or composition of thecomposite oxide semiconductor. For example, it is possible to form acomposite oxide semiconductor with a low proportion of Regions A1 or acomposite oxide semiconductor with a high proportion of Regions A1. In acomposite oxide semiconductor of one embodiment of the presentinvention, the proportion of Regions A1 is not always lower than that ofRegions B1. In a composite oxide semiconductor with an extremely highproportion of Regions A1, depending on the observation range, Region B1is sometimes formed in Region A1. The size of the particulate region ofRegion A1 can be appropriately adjusted by changing, for example, theformation conditions or composition of the composite oxidesemiconductor.

In some cases, the boundary of Region A1, the boundary of Region B1, andthe boundary of Region C1 are not clearly observed. The sizes of RegionA1, Region B1, and Region C1 can be measured with energy dispersiveX-ray spectroscopy (EDX) mapping images obtained by EDX. For example,the diameter of a cluster in Region Al is greater than or equal to 0.5nm and less than or equal to 10 nm in the EDX mapping image of across-sectional photograph or a plan-view photograph in some cases. Notethat the diameter of the cluster is preferably greater than or equal to1 nm and less than or equal to 2 nm.

As described above, an oxide semiconductor of one embodiment of thepresent invention is a composite oxide semiconductor in which Region A1and Region B1 are mixed and have different functions that arecomplementary to each other.

In contrast, when Region A1 and Region B1 are stacked in a layeredmanner, for example, interaction does not take place or is unlikely totake place between Region Al and Region B1, so that the function ofRegion A1 and that of Region B1 are independently performed in somecases. In that case, even when the carrier mobility is increased owingto Region A1, the off-state current of the transistor might beincreased. Therefore, in the case of using the composite oxidesemiconductor of one embodiment of the present invention, a function ofachieving high carrier mobility and a function of achieving excellentswitching characteristics can be obtained at the same time. This is anadvantageous effect obtained by using the composite oxide semiconductorof one embodiment of the present invention.

<4-4. Atomic Ratio of Composite Oxide Semiconductor>

The atomic ratio of elements included in the composite oxidesemiconductor of one embodiment of the present invention will bedescribed here.

Phase diagrams in FIGS. 21A to 21C can be used to show the atomic ratioof elements in the case where the composite oxide semiconductor containsIn, the element M, and Zn. The atomic ratio of In to the element M andZn is denoted by x:y:z. This atomic ratio can be shown as coordinates(x:y:z) in FIGS. 21A to 21C. Note that the proportion of oxygen atoms isnot illustrated in FIGS. 21A to 21C.

In FIGS. 21A to 21C, dashed lines correspond to a line representing theatomic ratio of [In]:[M]:[Zn]=(1+α):(1−α):1 (−1≤α≤1), a linerepresenting the atomic ratio of [In]:[M]:[Zn]=(1+α):(1−α):2, a linerepresenting the atomic ratio of [In]:[M]:[Zn]=(1+α):(1−α):3, a linerepresenting the atomic ratio of [In]:[M]:[Zn]=(1+a):(1−α):4, and a linerepresenting the atomic ratio of [In]:[M]:[Zn]=(1+a):(1−α):5.

Dashed-dotted lines correspond to a line representing the atomic ratioof [In]:[M]:[Zn]=1:1:β (β≥0), a line representing the atomic ratio of[In]:[M]:[Zn]=1:2:β, a line representing the atomic ratio of[In]:[M]:[Zn]=1:3:β, a line representing the atomic ratio of[In]:[M]:[Zn]=1:4:β, a line representing the atomic ratio of[In]:[M]:[Zn]=1:7:β, a line representing the atomic ratio of[In]:[M]:[Zn]=2:1:β, and a line representing the atomic ratio of[In]:[M]:[Zn]=5:1:β.

An oxide semiconductor having the atomic ratio of [In]:[M]:[Zn]=0:2:1 ora neighborhood thereof in FIGS. 21A to 21C tends to have a spinelcrystal structure.

A plurality of phases (e.g., two phases or three phases) exist in thecomposite oxide semiconductor in some cases. For example, with an atomicratio [In]:[M]:[Zn] that is close to 0:2:1, two phases of a spinelcrystal structure and a layered crystal structure are likely to exist.In addition, with an atomic ratio [In]:[M]:[Zn] that is close to 1:0:0,two phases of a bixbyite crystal structure and a layered crystalstructure are likely to exist. In the case where a plurality of phasesexist in the composite oxide semiconductor, a grain boundary might beformed between different crystal structures.

A region A in FIG. 21A represents examples of the preferred ranges ofthe atomic ratio of indium, the element M, and zinc contained in acomposite oxide semiconductor.

In addition, the composite oxide semiconductor containing indium in ahigher proportion can have high carrier mobility (electron mobility).Thus, a composite oxide semiconductor having a high content of indiumhas higher carrier mobility than an composite oxide semiconductor havinga low content of indium.

In contrast, when the indium content and the zinc content in a compositeoxide semiconductor become lower, carrier mobility becomes lower. Thus,with an atomic ratio of [In]:[M]:[Zn]=0:1:0 and the vicinity thereof(e.g., a region C in FIG. 21C), insulation performance becomes better.

Accordingly, a composite oxide semiconductor of one embodiment of thepresent invention preferably has an atomic ratio represented by theregion A in FIG. 21A. With the atomic ratio, a stacked-layer structurewith high carrier mobility and a few grain boundaries is easilyobtained.

A composite oxide semiconductor with an atomic ratio in the region A,particularly in a region B in FIG. 21B, is excellent because thecomposite oxide semiconductor easily becomes a CAAC-OS and has highcarrier mobility.

The CAAC-OS is a composite oxide semiconductor with high crystallinity.In contrast, in the CAAC-OS, a reduction in electron mobility due to thegrain boundary is less likely to occur because a clear grain boundarycannot be observed. Entry of impurities, formation of defects, or thelike might decrease the crystallinity of a composite oxidesemiconductor. This means that the CAAC-OS has small amounts ofimpurities and defects (e.g., oxygen vacancies). Thus, a composite oxidesemiconductor including a CAAC-OS is physically stable. Therefore, thecomposite oxide semiconductor including a CAAC-OS is resistant to heatand has high reliability.

Note that the region B includes an atomic ratio of [In]:[M]:[Zn]=4:2:3to 4:2:4.1 and the vicinity thereof. The vicinity includes an atomicratio of [In]:[M]:[Zn]=5:3:4. Note that the region B includes an atomicratio of [In]:[M]:[Zn]=5:1:6 and the vicinity thereof and an atomicratio of [In]:[M]:[Zn]=5:1:7 and the vicinity thereof.

Note that the property of a composite oxide semiconductor is notuniquely determined by an atomic ratio. Even with the same atomic ratio,the property of a composite oxide semiconductor might be differentdepending on a formation condition. For example, in the case where thecomposite oxide semiconductor is deposited with a sputtering apparatus,a film having an atomic ratio deviated from the atomic ratio of a targetis formed. In particular, [Zn] in the film might be smaller than [Zn] inthe target depending on the substrate temperature in deposition. Thus,the illustrated regions each represent an atomic ratio with which acomposite oxide semiconductor tends to have specific characteristics,and boundaries of the regions A to C are not clear.

<4-5. Method for Manufacturing Composite Oxide Semiconductor>

An example of a method for manufacturing the composite oxidesemiconductor shown in FIG. 20 is described. A composite oxidesemiconductor of one embodiment of the present invention can be formedwith a sputtering apparatus.

[Sputtering Apparatus]

FIG. 22A is a cross-sectional view of a deposition chamber 2501 of thesputtering apparatus. FIG. 22B is a plan view of a magnet unit 2530 aand a magnet unit 2530 b of the sputtering apparatus.

The deposition chamber 2501 illustrated in FIG. 22A includes a targetholder 2520 a, a target holder 2520 b, a backing plate 2510 a, a backingplate 2510 b, a target 2502 a, a target 2502 b, a member 2542, and asubstrate holder 2570. Note that the target 2502 a is placed over thebacking plate 2510 a. The backing plate 2510 a is placed over the targetholder 2520 a. The magnet unit 2530 a is placed under the target 2502 awith the backing plate 2510 a therebetween. The target 2502 b is placedover the backing plate 2510 b. The backing plate 2510 b is placed overthe target holder 2520 b. The magnet unit 2530 b is placed under thetarget 2502 b with the backing plate 2510 b therebetween.

As illustrated in FIGS. 22A and 22B, the magnet unit 2530 a includes amagnet 2530N1, a magnet 2530N2, a magnet 2530S, and a magnet holder2532. The magnet 2530N1, the magnet 2530N2, and the magnet 2530S areplaced over the magnet holder 2532 in the magnet unit 2530 a. The magnet2530N1, the magnet 2530N2, and the magnet 2530S are spaced. Note thatthe magnet unit 2530 b has a structure similar to that of the magnetunit 2530 a. When the substrate 2560 is transferred into the depositionchamber 2501, the substrate 2560 is placed in contact with the substrateholder 2570.

The target 2502 a, the backing plate 2510 a, and the target holder 2520a are separated from the target 2502 b, the backing plate 2510 b, andthe target holder 2520 b by the member 2542. Note that the member 2542is preferably an insulator. The member 2542 may be a conductor or asemiconductor. The member 2542 may be a conductor or a semiconductorwhose surface is covered with an insulator.

The target holder 2520 a and the backing plate 2510 a are fixed to eachother with a screw (e.g., a bolt) and have the same potential. Thetarget holder 2520 a has a function of supporting the target 2502 a withthe backing plate 2510 a positioned therebetween. The target holder 2520b and the backing plate 2510 b are fixed to each other with a screw(e.g., a bolt) and have the same potential. The target holder 2520 b hasa function of supporting the target 2502 b with the backing plate 2510 bpositioned therebetween.

The backing plate 2510 a has a function of fixing the target 2502 a. Thebacking plate 2510 b has a function of fixing the target 2502 b.

Magnetic lines of force 2580 a and 2580 b formed by the magnet unit 2530a are illustrated in FIG. 22A.

As illustrated in FIG. 22B, the magnet unit 2530 a has a structure inwhich the magnet 2530N1 having a rectangular or substantiallyrectangular shape, the magnet 2530N2 having a rectangular orsubstantially rectangular shape, and the magnet 2530S having arectangular or substantially rectangular shape are fixed to the magnetholder 2532. The magnet unit 2530 a can be oscillated horizontally asshown by an arrow in FIG. 22B. For example, the magnet unit 2530 a maybe oscillated with a beat of greater than or equal to 0.1 Hz and lessthan or equal to 1 kHz.

The magnetic field over the target 2502 a changes in accordance withoscillation of the magnet unit 2530 a. The region with an intensemagnetic field is a high-density plasma region; thus, sputtering of thetarget 2502 a easily occurs in the vicinity of the region. The sameapplies to the magnet unit 2530 b.

<4-6. Formation Method Composite Oxide Semiconductor>

Next, a formation method of a composite oxide semiconductor isdescribed. FIG. 23 is a process flow chart showing the manufacturingmethod of a composite oxide semiconductor.

A composite oxide semiconductor shown in FIG. 20 is formed through atleast first to fourth steps shown in FIG. 23.

[First Process: Process of Placing Substrate in Deposition Chamber]

The first process includes a step of placing a substrate in a depositionchamber (see Step S102 in FIG. 23).

In the first process, for example, the substrate 2560 is placed on thesubstrate holder 2570 of the deposition chamber 2501 shown in FIGS. 22Aand 22B.

The temperature of the substrate 2560 in deposition influences theelectrical properties of a composite oxide semiconductor. The higher thesubstrate temperature is, the higher the crystallinity and reliabilityof the composite oxide semiconductor can be. In contrast, the lower thesubstrate temperature is, the lower the crystallinity of the compositeoxide semiconductor can be and the higher the carrier mobility thereofcan be. In particular, the lower the substrate temperature in depositionis, the more the field-effect mobility at a low gate voltage (e.g.,higher than 0 V and lower than or equal to 2 V) is notably increased ina transistor including the composite oxide semiconductor.

The temperature of the substrate 2560 is higher than or equal to roomtemperature (25° C.) and lower than or equal to 200° C., preferablyhigher than or equal to room temperature and lower than or equal to 170°C., further preferably higher than or equal to room temperature andlower than or equal to 130° C. Setting the substrate temperature in theabove range is favorable for large glass substrates (e.g., theabove-described 8th- to 10th-generation glass substrates). Inparticular, when the substrate temperature in deposition of a compositeoxide semiconductor is room temperature, i.e., the substrate is notheated intentionally, the substrate can be favorably prevented frombending or warping.

The substrate 2560 may be cooled with a cooling mechanism or the likeprovided for the substrate holder 2570.

When the temperature of the substrate 2560 is higher than or equal to100° C. and lower than or equal to 130° C., water in the composite oxidesemiconductor can be removed. Removing water, which is an impurity, insuch a manner leads to high field-effect mobility and high reliability.

The temperature of the substrate 2560 is set to higher than or equal to100° C. and lower than or equal to 130° C. for removal of water, wherebythe sputtering apparatus can be prevented from warping due to overheat.This leads to an improvement in productivity of a semiconductor device.The productivity is stabilized, so that a large-scale productionapparatus is easy to employ. Thus, a large display device manufacturedusing a large substrate can be easily manufactured.

When the temperature of the substrate 2560 is high, water in thecomposite oxide semiconductor can be more effectively removed andmoreover, the composite oxide semiconductor can have increasedcrystallinity. For example, the temperature of the substrate 2560 is setto higher than or equal to 80° C. and lower than or equal to 200° C.,preferably higher than or equal to 100° C. and lower than or equal to170° C., whereby a composite oxide semiconductor with high crystallinitycan be formed.

[Second Process: Process of Introducing Gas into Deposition Chamber]

The second step includes a step of introducing gas into the depositionchamber (see step S202 in FIG. 23).

In the second step, for example, gas is introduced into the depositionchamber 2501 shown in FIGS. 22A and 22B. An argon gas and/or an oxygengas are/is introduced as the gas. Note that instead of an argon gas, aninert gas such as helium, xenon, or krypton can be used.

The oxygen flow rate in forming a composite oxide semiconductor using anoxygen gas shows the following tendency. The higher the oxygen flow rateis, the higher the crystallinity of the composite oxide semiconductor isand the higher the reliability is. In contrast, the lower the oxygenflow rate is, the lower the crystallinity of the composite oxidesemiconductor is and the higher the carrier mobility is. Specifically,in a transistor including the composite oxide semiconductor, the smallerthe oxygen flow rate is, the more notably the field-effect mobility at alow gate voltage (e.g., higher than 0 V and lower than or equal to 2 V)is increased.

The oxygen flow rate ratio can be appropriately set in the range from 0%to 100% inclusive so that favorable characteristics of the compositeoxide semiconductor suitable to the uses can be obtained.

For example, in the case where the composite oxide semiconductor is usedfor a semiconductor layer of a transistor having high field-effectmobility, the oxygen flow rate is set to higher than or equal to 0% andlower than or equal to 30%, preferably higher than or equal to 5% andlower than or equal to 30%, further preferably higher than or equal to7% and lower than or equal to 15% in deposition of the composite oxidesemiconductor.

To make both the field-effect mobility and reliability of a transistorhigh, the oxygen flow rate ratio in deposition of the composite oxidesemiconductor is set to higher than 30% and lower than 70%, preferablyhigher than 30% and lower than or equal to 50%. Alternatively, theoxygen flow rate ratio in deposition of the composite oxidesemiconductor is set to higher than or equal to 10% and lower than orequal to 50%, preferably higher than 30% and lower than or equal to 50%.

To make the reliability of a transistor high, the oxygen flow rate ratioin deposition of the composite oxide semiconductor is set to higher thanor equal to 70% and lower than or equal to 100%.

When the substrate temperature and the oxygen flow rate ratio indeposition are controlled in this manner, a composite oxidesemiconductor that provides desired electrical characteristics can bedeposited. For example, a reduction (an increase) of substratetemperature and a reduction (an increase) of oxygen flow rate ratiocontribute to the field-effect mobility to the same degree, in somecases. Therefore, even when the substrate temperature cannot beincreased sufficiently with the constraints of an apparatus, forexample, a transistor having field-effect mobility substantially thesame as that when the substrate temperature is increased can be obtainedby increasing the oxygen flow rate ratio.

Impurities in the oxide semiconductor film are reduced by the method inEmbodiment 1, whereby a highly reliable transistor can be obtained.

[Third Process: Process of Applying Voltage to Target]

The third process includes a step of applying voltage to a target (seeStep S302 in FIG. 23).

In the third process, for example, voltage is applied to the targetholder 2520 a and the target holder 2520 b in FIGS. 22A and 22B. As anexample, a potential applied to a terminal V1 connected to the targetholder 2520 a is lower than a potential applied to a terminal V2connected to the substrate holder 2570. A potential applied to aterminal V4 connected to the target holder 2520 b is lower than thepotential applied to the terminal V2 connected to the substrate holder2570. The potential applied to the terminal V2 connected to thesubstrate holder 2570 is a ground potential. A potential applied to aterminal V3 connected to the magnet holder 2532 is a ground potential.

Note that the potentials applied to the terminals V1, V2, V3, and V4 arenot limited to the above potentials. Not all the target holder 2520, thesubstrate holder 2570, and the magnet holder 2532 are necessarilysupplied with potentials. For example, the substrate holder 2570 may beelectrically floating. Note that it is assumed that a power sourcecapable of controlling a potential applied to the terminal V1 iselectrically connected to the terminal V1. As the power source, a DCpower source, an AC power source, or an RF power source may be used.

As the target 2502 a and the target 2502 b, a target including indium,the element M (M is Ga, Al, Y, or Sn), zinc, and oxygen is preferablyused. For example, an In—Ga—Zn metal oxide target (In:Ga:Zn=4:2:4.1[atomic ratio]) or an In—Ga—Zn metal oxide target (In:Ga:Zn=5:1:7[atomic ratio]) can be used as the target 2502 a and the target 2502 b.In the following description, the case of using an In—Ga—Zn metal oxidetarget (In:Ga:Zn=4:2:4.1 [atomic ratio]) is described.

[Fourth Process: Process of Depositing Composite Oxide Semiconductor onSubstrate]

The fourth process includes a step in which sputtered particles areejected from the target and a composite oxide semiconductor is depositedon the substrate (see Step S402 in FIG. 23).

In the fourth process, for example, in the deposition chamber 2501illustrated in FIGS. 22A and 22B, an argon gas or an oxygen gas isionized to be separated into cations and electrons, and plasma iscreated. Then, the cations in the plasma are accelerated toward thetargets 2502 a and 2502 b by the potentials applied to the targetholders 2520 a and 2520 b. Sputtered particles are generated when thecations collide with the In—Ga—Zn metal oxide target, and the sputteredparticles are deposited on the substrate 2560.

Note that in an In—Ga—Zn metal oxide target with an atomic ratio ofIn:Ga:Zn=4:2:4.1 or 5:1:7 that is used as the targets 2502 a and 2502 b,a plurality of crystal grains with different compositions are includedin some cases. In most cases, for example, the diameters of theplurality of crystal grains are each 10 μm or less. In the case where,for example, crystal grains with a high proportion of In are included inthe In—Ga—Zn metal oxide target, the proportion of Region A1 describedabove is increased in some cases.

<4-7. Deposition Model>

In the fourth process, a deposition model shown in FIGS. 24A to 24C canbe presumed.

FIGS. 24A to 24C are cross-sectional views of the vicinity of the target2502 a shown in FIGS. 22A and 22B. Note that FIG. 24A shows the state ofthe target before use, FIG. 24B shows the state of the target beforedeposition, and FIG. 24C shows the state of the target during thedeposition. In FIGS. 24A to 24C, the target 2502 a, plasma 2190, cations2192, sputtered particles 2504 a and 2506 a, and the like are shown.

In FIG. 24A, a surface of the target 2502 a is relatively flat and itscomposition (e.g., the composition ratio between In, Ga, and Zn) isuniform. In contrast, in FIG. 24B, unevenness is formed on the surfaceof the target 2502 a by sputtering treatment performed in advance or thelike, and compositional segregation occurs. The unevenness and thesegregation can occur because of plasma (e.g., Ar plasma) generated inthe sputtering treatment performed in advance. Note that FIG. 24Billustrates a segregation region 2504 and a segregation region 2506.Here, the segregation region 2504 is a region containing a large amountof Ga and a large amount of Zn (a Ga,Zn-rich region), and thesegregation region 2506 is a region containing a large amount of In (anIn-rich region). The segregation region 2504, which contains a largeamount of Ga, is formed because the melting point of Ga lower than thatof In allows part of Ga to be melted by heat applied to the target 2502a during the plasma treatment and aggregate.

[First Step]

In FIG. 24C, an argon gas or an oxygen gas is separated into the cation2192 and an electron (not illustrated), and the plasma 2190 is created.After that, the cations 2192 in the plasma 2190 are accelerated towardthe target 2502 a (here, an In—Ga—Zn oxide target). The cations 2192collide with the In—Ga—Zn oxide target, whereby the sputtered particles2504 a and 2506 a are generated and ejected from the In—Ga—Zn oxidetarget. Note that since the sputtered particles 2504 a are ejected fromthe segregation region 2504, they form a Ga,Zn-rich cluster in somecases. Since the sputtered particles 2506 a are ejected from thesegregation region 2506, they form an In-rich cluster in some cases.

When an In—Ga—Zn oxide target is used, presumably, the sputteredparticles 2504 a are preferentially sputtered first from the segregationregion 2504. This is because Ga and Zn, which have lower relative atomicmasses than In, are preferentially ejected from the In—Ga—Zn oxidetarget by collision of the cation 2192 with the In—Ga—Zn oxide target.The sputtered particles 2504 a are deposited over the substrate, therebyforming Region B1 illustrated in FIG. 20.

[Second Step]

Next, as illustrated in FIG. 24C, the sputtered particles 2506 a aresputtered from the segregation region 2506. The sputtered particles 2506a collide with Region B1 that has been formed over the substrate,thereby forming Region A1 illustrated in FIG. 20.

As illustrated in FIG. 24C, the target 2502 a is subjected to sputteringthroughout the deposition; thus, generation of the segregation region2504 and disappearance of the segregation region 2504 occurintermittently.

The deposition model including the first step and the second step isrepeated, whereby the composite oxide semiconductor that is oneembodiment of the present invention and shown in FIG. 20 can beobtained.

That is, the sputtered particles (2506 a and 2504 a) are respectivelyejected from the In-rich segregation region 2506 and the Ga,Zn-richsegregation region 2504 to be deposited over the substrate. The In-richregions are connected to each other in a cloud-like manner over thesubstrate, so that a composite oxide semiconductor of one embodiment ofthe present invention as illustrated in FIGS. 19A and 19B can be formed.In a film of the composite oxide semiconductor, the In-rich regions areconnected to each other in a cloud-like manner. Thanks to this, atransistor using the composite oxide semiconductor has a high on-statecurrent (I_(on)) and high field-effect mobility (μFE).

In this manner, for a transistor having a high on-state current (I_(on))and high field-effect mobility (μFE), In is of importance and othermetals (e.g., Ga) are not always necessary.

Note that described above as an example is a model in which a compositeoxide semiconductor is formed with the use of an argon gas. In thatcase, the composite oxide semiconductor might contain many oxygenvacancies. When the composite oxide semiconductor contains many oxygenvacancies, shallow defect states (also referred to as sDOS) are formedin the composite oxide semiconductor in some cases. When sDOS is formedin the composite oxide semiconductor, the sDOS serves as a carrier trap,resulting in a reduction in on-state current and field-effect mobility.

Therefore, when a composite oxide semiconductor is formed with the useof an argon gas, it is preferable to supply oxygen into the compositeoxide semiconductor after formation thereof so that oxygen vacancies inthe composite oxide semiconductor are compensated and sDOS is reduced.

Such oxygen supply can be conducted by, for example, performing heattreatment in an atmosphere that contains oxygen after the formation ofthe composite oxide semiconductor, or performing plasma treatment in anatmosphere that contains oxygen. Alternatively, either an insulatingfilm that is in contact with the composite oxide semiconductor or aninsulating film in the vicinity of the composite oxide semiconductor ismade to contain excess oxygen. The mode in which such an insulating filmcontains excess oxygen are described in Embodiment 2.

Note that one embodiment of the present invention is not limited to theabove-mentioned manufacturing method by a sputtering method. A pulsedlaser deposition (PLD) method, a plasma-enhanced chemical vapordeposition (PECVD) method, a thermal chemical vapor deposition (CVD)method, an atomic layer deposition (ALD) method, a vacuum evaporationmethod, or the like may be used. As an example of a thermal CVD method,a metal organic chemical vapor deposition (MOCVD) method can be given.

<4-8. Transistor Including Oxide Semiconductor Film>

Next, a transistor including an oxide semiconductor film is described.

Note that when the composite oxide semiconductor is used for atransistor, the transistor can have high carrier mobility and excellentswitching characteristics. In addition, the transistor can have highreliability.

An oxide semiconductor film with a low carrier density is preferablyused for a transistor. For example, an oxide semiconductor film whosecarrier density is lower than 8×10¹¹ /cm³, preferably lower than 1×10¹¹/cm³, or further preferably lower than 1×10¹⁰ /cm³, and greater than orequal to 1×10⁻⁹ /cm³ is used as the oxide semiconductor film.

An oxide semiconductor film of one embodiment of the present inventionis a highly purified intrinsic or substantially highly purifiedintrinsic oxide semiconductor film as described above. A highly purifiedintrinsic or substantially highly purified intrinsic oxide semiconductorfilm has few carrier generation sources, and thus can have a low carrierdensity. A highly purified intrinsic or substantially highly purifiedintrinsic oxide semiconductor film has a low density of defect statesand accordingly has a low density of trap states in some cases.

Charges trapped by the trap states in the oxide semiconductor film takea long time to be released and may behave like fixed charges. Thus, thetransistor whose channel region is formed in the oxide semiconductorhaving a high density of trap states has unstable electricalcharacteristics in some cases.

To obtain stable electrical characteristics of the transistor, it iseffective to reduce the concentration of impurities in the oxidesemiconductor film. In order to reduce the concentration of impuritiesin the oxide semiconductor film, the concentration of impurities in afilm that is adjacent to the oxide semiconductor film is preferablyreduced. As examples of the impurities, hydrogen, nitrogen, alkalimetal, alkaline earth metal, iron, nickel, silicon, and the like aregiven.

Here, the influence of impurities in the oxide semiconductor film willbe described.

When silicon or carbon that is one of Group 14 elements is contained inthe oxide semiconductor film, defect states are formed in the oxidesemiconductor. Thus, the concentration of silicon or carbon in the oxidesemiconductor and around an interface with the oxide semiconductor(measured by secondary ion mass spectrometry (SIMS)) is set lower thanor equal to 2×10¹⁸ atoms/cm³, and preferably lower than or equal to2×10¹⁷ atoms/cm³.

When the oxide semiconductor film contains alkali metal or alkalineearth metal, defect states are formed and carriers are generated, insome cases. Thus, a transistor including an oxide semiconductor filmwhich contains alkali metal or alkaline earth metal is likely to benormally on. Therefore, it is preferable to reduce the concentration ofalkali metal or alkaline earth metal in the oxide semiconductor film.Specifically, the concentration of alkali metal or alkaline earth metalin the oxide semiconductor film measured by SIMS is set lower than orequal to 1×10¹⁸ atoms/cm³, and preferably lower than or equal to 2×10¹⁶atoms/cm³.

When the oxide semiconductor film contains nitrogen, the oxide easilybecomes n-type by generation of electrons serving as carriers and anincrease of carrier density. Thus, a transistor whose semiconductorincludes an oxide semiconductor that contains nitrogen is likely to benormally-on. For this reason, nitrogen in the oxide semiconductor ispreferably reduced as much as possible; the nitrogen concentrationmeasured by SIMS is set, for example, lower than 5×10¹⁹ atoms/cm³,preferably lower than or equal to 5×10¹⁸ atoms/cm³, further preferablylower than or equal to 1×10¹⁸ atoms/cm³, and still further preferablylower than or equal to 5×10¹⁷ atoms/cm³.

Hydrogen contained in an oxide semiconductor film reacts with oxygenbonded to a metal atom to be water, and thus causes an oxygen vacancy(V_(o)), in some cases. Due to entry of hydrogen into the oxygen vacancy(V_(o)), an electron serving as a carrier is generated in some cases. Insome cases, bonding of part of hydrogen to oxygen bonded to a metal atomcauses generation of an electron serving as a carrier. Thus, atransistor including an oxide semiconductor which contains hydrogen islikely to be normally on. Accordingly, it is preferable that hydrogen inthe oxide semiconductor be reduced as much as possible. Specifically,the hydrogen concentration measured by SIMS is set lower than 1×10²⁰atoms/cm³, preferably lower than 1×10¹⁹ atoms/cm³, further preferablylower than 5×10¹⁸ atoms/cm³, and still further preferably lower than1×10¹⁸ atoms/cm³.

Note that oxygen vacancies (V_(o)) in the oxide semiconductor film canbe reduced by introduction of oxygen into the oxide semiconductor. Thatis, the oxygen vacancies (V_(o)) in the oxide semiconductor filmdisappear when the oxygen vacancies (V_(o)) are filled with oxygen.Accordingly, diffusion of oxygen into the oxide semiconductor film canreduce the amount of oxygen vacancies (V_(o)) in a transistor andimprove the reliability of the transistor.

As a method for introducing oxygen into the oxide semiconductor film,for example, an oxide in which oxygen content is higher than that in thestoichiometric composition is provided in contact with the oxidesemiconductor. That is, in the oxide, a region including oxygen inexcess of that in the stoichiometric composition (hereinafter alsoreferred to as an excess oxygen region) is preferably formed. Inparticular, in the case of using an oxide semiconductor film in atransistor, an oxide including an excess-oxygen region is provided in abase film, an interlayer film, or the like in the vicinity of thetransistor, whereby oxygen vacancies in the transistor are reduced, andthe reliability can be improved.

When an oxide semiconductor film with sufficiently reduced impurityconcentration is used for a channel formation region in a transistor,the transistor can have stable electrical characteristics.

At least part of this embodiment can be implemented in combination withany of the other embodiments described in this specification asappropriate.

Embodiment 5

In this embodiment, examples of a display device that includes thetransistor described in the above embodiments are described below withreference to FIG. 25 to FIG. 27.

FIG. 25 is a top view illustrating an example of a display device. Adisplay device 700 in FIG. 25 includes a pixel portion 702 provided overa first substrate 701, a source driver circuit portion 704 and a gatedriver circuit portion 706 that are provided over the first substrate701, a sealant 712 provided to surround the pixel portion 702, thesource driver circuit portion 704, and the gate driver circuit portion706, and a second substrate 705 provided to face the first substrate701. The first substrate 701 and the second substrate 705 are sealedwith the sealant 712. That is, the pixel portion 702, the source drivercircuit portion 704, and the gate driver circuit portion 706 areenclosed by the first substrate 701, the sealant 712, and the secondsubstrate 705. Although not illustrated in FIG. 25, a display element isprovided between the first substrate 701 and the second substrate 705.

In the display device 700, a flexible printed circuit (FPC) terminalportion 708 that is electrically connected to the pixel portion 702, thesource driver circuit portion 704, and the gate driver circuit portion706 is provided in a region different from the region that is over thefirst substrate 701 and surrounded by the sealant 712. Furthermore, anFPC 716 is connected to the FPC terminal portion 708, and a variety ofsignals and the like are supplied from the FPC 716 to the pixel portion702, the source driver circuit portion 704, and the gate driver circuitportion 706. Furthermore, a signal line 710 is connected to the pixelportion 702, the source driver circuit portion 704, the gate drivercircuit portion 706, and the FPC terminal portion 708. Through thesignal line 710, a variety of signals and the like are supplied from theFPC 716 to the pixel portion 702, the source driver circuit portion 704,the gate driver circuit portion 706, and the FPC terminal portion 708.

A plurality of gate driver circuit portions 706 may be provided in thedisplay device 700. The structure of the display device 700 is notlimited to the example shown here, in which the source driver circuitportion 704 and the gate driver circuit portion 706 as well as the pixelportion 702 are formed over the first substrate 701. For example, onlythe gate driver circuit portion 706 may be formed over the firstsubstrate 701, or only the source driver circuit portion 704 may beformed over the first substrate 701. In this case, a substrate overwhich a source driver circuit, a gate driver circuit, or the like isformed (e.g., a driver circuit board formed using a single crystalsemiconductor film or a polycrystalline semiconductor film) may beformed on the first substrate 701. Note that there is no particularlimitation on the method for connecting the separately prepared drivercircuit board, and a chip on glass (COG) method, a wire bonding method,or the like can be used.

The pixel portion 702, the source driver circuit portion 704, and thegate driver circuit portion 706 included in the display device 700include a plurality of transistors. The transistor which is asemiconductor device of one embodiment of the present invention can beused.

The display device 700 can include a variety of elements. As examples ofthe elements, electroluminescent (EL) element (e.g., an EL elementcontaining organic and inorganic materials, an organic EL element, aninorganic EL element, or an LED), a light-emitting transistor element (atransistor that emits light depending on current), an electron emitter,a liquid crystal element, an electronic ink display, an electrophoreticelement, an electrowetting element, a plasma display panel (PDP), microelectro mechanical systems (MEMS) display (e.g., a grating light valve(GLV), a digital micromirror device (DMD), a digital micro shutter (DMS)element, or an interferometric modulator display (IMOD) element), apiezoelectric ceramic display, and the like can be given.

An example of a display device including an EL element is an EL display.Examples of a display device including an electron emitter include afield emission display (FED) and an SED-type flat panel display (SED:surface-conduction electron-emitter display). An example of a displaydevice including a liquid crystal element is a liquid crystal display (atransmissive liquid crystal display, a transflective liquid crystaldisplay, a reflective liquid crystal display, a direct-view liquidcrystal display, or a projection liquid crystal display). An example ofa display device including an electronic ink display or anelectrophoretic element is electronic paper. In a transflective liquidcrystal display or a reflective liquid crystal display, some or all ofpixel electrodes may function as reflective electrodes. For example,some or all of pixel electrodes may contain aluminum, silver, or thelike. In this case, a memory circuit such as an SRAM can be providedunder the reflective electrodes, leading to lower power consumption.

As a display system of the display device 700, a progressive system, aninterlace system, or the like can be employed. Furthermore, colorelements controlled in pixels at the time of color display are notlimited to three colors: R, G, and B (R, G, and B correspond to red,green, and blue, respectively). For example, four pixels of an R pixel,a G pixel, a B pixel, and a W (white) pixel may be used. Alternatively,a color element may be composed of two colors of R, G, and B as inPenTile layout. The two colors may differ depending on the colorelements. Alternatively, one or more colors of yellow, cyan, magenta,and the like may be added to RGB. Note that the size of a display regionmay differ between dots of color elements. One embodiment of thedisclosed invention is not limited to a color display device; thedisclosed invention can also be applied to a monochrome display device.

A coloring layer (also referred to as a color filter) may be used toobtain a full-color display device in which white light (W) is used fora backlight (e.g., an organic EL element, an inorganic EL element, anLED, or a fluorescent lamp). For example, a red (R) coloring layer, agreen (G) coloring layer, a blue (B) coloring layer, and a yellow (Y)coloring layer can be combined as appropriate. With the use of thecoloring layer, high color reproducibility can be obtained as comparedwith the case without the coloring layer. Here, by providing a regionwith a coloring layer and a region without a coloring layer, white lightin the region without the coloring layer may be directly utilized fordisplay. By partly providing the region without a coloring layer, adecrease in the luminance of a bright image due to the coloring layercan be suppressed, and power consumption can be reduced by approximately20% to 30% in some cases. In the case where full-color display isperformed using a self-luminous element such as an organic EL element oran inorganic EL element, elements may emit light in their respectivecolors R, G, B, Y, and W. By using a self-luminous element, powerconsumption may be further reduced as compared with the case of using acoloring layer.

As a coloring system, any of the following systems may be used: theabove-described color filter system in which part of white light isconverted into red light, green light, and blue light through colorfilters; a three-color system in which red light, green light, and bluelight are used; and a color conversion system or a quantum dot system inwhich part of blue light is converted into red light or green light.

In this embodiment, a structure including a liquid crystal element as adisplay element and a structure including an EL element as a displayelement are described with reference to FIG. 26 and FIG. 27. FIG. 26 isa cross-sectional view taken along dashed-dotted line Q-R in FIG. 25 andillustrate the structure including a liquid crystal element as a displayelement. FIG. 27 is a cross-sectional view taken along dashed-dottedline Q-R in FIG. 25 and illustrates the structure including an ELelement as a display element.

Portions common to FIG. 26 and FIG. 27 are described first, and then,different portions are described.

<5-1. Portions Common to Display Devices>

The display device 700 in FIG. 26 and FIG. 27 includes a lead wiringportion 711, the pixel portion 702, the source driver circuit portion704, and the FPC terminal portion 708. The lead wiring portion 711includes the signal line 710. The pixel portion 702 includes atransistor 750 and a capacitor 790. The source driver circuit portion704 includes a transistor 752.

The transistor 750 and the transistor 752 each have a structure similarto that of the transistor 100D described above. Note that the transistor750 and the transistor 752 may each have the structure of any of theother transistors described in the above embodiments.

The transistor used in this embodiment includes an oxide semiconductorfilm that is highly purified and in which formation of oxygen vacanciesis inhibited. The transistor can have a low off-state current.Accordingly, an electrical signal such as an image signal can be heldfor a long time, and a long writing interval can be set in an on state.Accordingly, the frequency of refresh operation can be reduced, whichsuppresses power consumption.

In addition, the transistor used in this embodiment can have relativelyhigh field-effect mobility and thus is capable of high-speed operation.For example, in a liquid crystal display device that includes such atransistor capable of high-speed operation, a switching transistor in apixel portion and a driver transistor in a driver circuit portion can beformed over one substrate. That is, no additional semiconductor deviceformed using a silicon wafer or the like is needed as a driver circuit;therefore, the number of components of the semiconductor device can bereduced. In addition, by using the transistor capable of high-speedoperation in the pixel portion, a high-quality image can be provided.

The capacitor 790 includes a lower electrode and an upper electrode. Thelower electrode is formed through a step of processing a conductive filmto be a conductive film functioning as a first gate electrode of thetransistor 750. The upper electrode is formed through a step ofprocessing a conductive film to be a conductive film functioning assource and drain electrodes of the transistor 750. Between the lowerelectrode and the upper electrode, an insulating film formed through astep of forming an insulating film to be an insulating film functioningas a first gate insulating film of the transistor 750 is provided. Thatis, the capacitor 790 has a stacked-layer structure in which aninsulating film functioning as a dielectric film is positioned betweenthe pair of electrodes.

In FIG. 26 and FIG. 27, a planarization insulating film 770 is providedover the transistor 750, the transistor 752, and the capacitor 790.

The planarization insulating film 770 can be formed using aheat-resistant organic material such as a polyimide resin, an acrylicresin, a polyimide amide resin, a benzocyclobutene resin, a polyamideresin, or an epoxy resin. Note that the planarization insulating film770 may be formed by stacking a plurality of insulating films formedfrom these materials. Alternatively, a structure without theplanarization insulating film 770 may be employed.

Although FIG. 26 and FIG. 27 each illustrate an example in which thetransistor 750 included in the pixel portion 702 and the transistor 752included in the source driver circuit portion 704 have the samestructure, one embodiment of the present invention is not limitedthereto. For example, the pixel portion 702 and the source drivercircuit portion 704 may include different transistors. Specifically, astructure in which a staggered transistor is used in the pixel portion702 and an inverted-staggered transistor described in Embodiment 1 isused in the source driver circuit portion 704, or a structure in whichthe inverted-staggered transistor described in Embodiment 1 is used inthe pixel portion 702 and the staggered transistor is used in the sourcedriver circuit portion 704 may be employed. Note that the term “sourcedriver circuit portion 704” can be replaced by the term “gate drivercircuit portion.”

The signal line 710 is formed through the same process as the conductivefilms functioning as source electrodes and drain electrodes of thetransistors 750 and 752. In the case where the signal line 710 is formedusing a material containing a copper element, signal delay or the likedue to wiring resistance is reduced, which enables display on a largescreen.

The FPC terminal portion 708 includes a connection electrode 760, ananisotropic conductive film 780, and the FPC 716. Note that theconnection electrode 760 is formed through the same process as theconductive films functioning as source electrodes and drain electrodesof the transistors 750 and 752. The connection electrode 760 iselectrically connected to a terminal included in the FPC 716 through theanisotropic conductive film 780.

For example, glass substrates can be used as the first substrate 701 andthe second substrate 705. As the first substrate 701 and the secondsubstrate 705, flexible substrates may also be used. An example of theflexible substrate is a plastic substrate.

A structure 778 is provided between the first substrate 701 and thesecond substrate 705. The structure 778 is a columnar spacer obtained byselective etching of an insulating film and is provided to control thedistance (cell gap) between the first substrate 701 and the secondsubstrate 705. Alternatively, a spherical spacer may also be used as thestructure 778.

A light-blocking film 738 functioning as a black matrix, a coloring film736 functioning as a color filter, and an insulating film 734 in contactwith the light-blocking film 738 and the coloring film 736 are providedon the second substrate 705 side.

<5-2. Structure Example of Input/Output Device of Display Device>

In the display device 700 shown in FIG. 26 and FIG. 27, a touch panel791 as an input/output device is provided. Note that the display device700 that does not include the touch panel 791 may be used.

The touch panel 791 illustrated in FIG. 26 and FIG. 27 is what is calledan in-cell touch panel provided between the second substrate 705 and thecoloring film 736. The touch panel 791 is formed on the second substrate705 side before the coloring film 736 is formed.

The touch panel 791 includes the light-blocking film 738, an insulatingfilm 792, an electrode 793, an electrode 794, an insulating film 795, anelectrode 796, and an insulating film 797. A change in the mutualcapacitance between the electrode 793 and the electrode 794 can besensed when an object such as a finger or a stylus approaches, forexample.

A portion in which the electrode 793 intersects with the electrode 794is illustrated in the upper portion of the transistor 750 illustrated inFIG. 26 and FIG. 27. The electrode 796 is electrically connected to thetwo electrodes 793 between which the electrode 794 is sandwiched throughopenings provided in the insulating film 795. Note that a structure inwhich a region where the electrode 796 is provided is provided in thepixel portion 702 is illustrated in FIG. 26 and FIG. 27 as an example;however, one embodiment of the present invention is not limited thereto.For example, the region where the electrode 796 is provided may beprovided in the source driver circuit portion 704.

The electrode 793 and the electrode 794 are provided in a regionoverlapping with the light-blocking film 738. As illustrated in FIG. 26,it is preferable that the electrode 793 not overlap with thelight-emitting element 782. As illustrated in FIG. 27, it is preferablethat the electrode 793 not overlap with the liquid crystal element 775.In other words, the electrode 793 has an opening in a region overlappingwith the light-emitting element 782 and the liquid crystal element 775.That is, the electrode 793 has a mesh shape. With such a structure, theelectrode 793 does not block light emitted from the light-emittingelement 782, or alternatively the electrode 793 does not block lighttransmitted through the liquid crystal element 775. Thus, sinceluminance is hardly reduced even when the touch panel 791 is provided, adisplay device with high visibility and low power consumption can beobtained. Note that the electrode 794 can have a structure similar tothat of the electrode 793.

Since the electrode 793 and the electrode 794 do not overlap with thelight-emitting element 782, a metal material having low transmittancewith respect to visible light can be used for the electrode 793 and theelectrode 794. Furthermore, since the electrode 793 and the electrode794 do not overlap with the liquid crystal element 775, a metal materialhaving low transmittance with respect to visible light can be used forthe electrode 793 and the electrode 794.

Thus, as compared with the case of using an oxide material whosetransmittance of visible light is high, resistance of the electrodes 793and 794 can be reduced, whereby sensitivity of the sensor of the touchpanel can be increased.

For example, a conductive nanowire may be used for the electrodes 793,794, and 796. The nanowire may have a mean diameter of greater than orequal to 1 nm and less than or equal to 100 nm, preferably greater thanor equal to 5 nm and less than or equal to 50 nm, further preferablygreater than or equal to 5 nm and less than or equal to 25 nm. As thenanowire, a carbon nanotube or a metal nanowire such as an Ag nanowire,a Cu nanowire, or an Al nanowire may be used. For example, in the casewhere an Ag nanowire is used for any one of or all of electrodes 664,665, and 667, the transmittance of visible light can be greater than orequal to 89% and the sheet resistance can be greater than or equal to 40Ω/sq. and less than or equal to 100 Ω/sq.

Although the structure of the in-cell touch panel is illustrated in FIG.26 and FIG. 27, one embodiment of the present invention is not limitedthereto. For example, a touch panel formed over the display device 700,what is called an on-cell touch panel, or a touch panel attached to thedisplay device 700, what is called an out-cell touch panel may be used.In this manner, the display device 700 of one embodiment of the presentinvention can be combined with various types of touch panels.

<5-3. Display Device Including Light-Emitting Element>

The display device 700 illustrated in FIG. 26 includes a light-emittingelement 782. The light-emitting element 782 includes a conductive film772, an EL layer 786, and a conductive film 788. The display device 700illustrated in FIG. 26 can display an image by utilizing light emissionfrom the EL layer 786 of the light-emitting element 782. Note that theEL layer 786 contains an organic compound or an inorganic compound suchas a quantum dot.

Examples of materials that can be used for an organic compound include afluorescent material and a phosphorescent material. Examples ofmaterials that can be used for a quantum dot include a colloidal quantumdot material, an alloyed quantum dot material, a core-shell quantum dotmaterial, and a core quantum dot material. A material containingelements belonging to Groups 12 and 16, elements belonging to Groups 13and 15, or elements belonging to Groups 14 and 16, may be used.Alternatively, a quantum dot material containing an element such ascadmium (Cd), selenium (Se), zinc (Zn), sulfur (S), phosphorus (P),indium (In), tellurium (Te), lead (Pb), gallium (Ga), arsenic (As), oraluminum (Al) may be used.

In the display device 700 in FIG. 26, the insulating film 730 isprovided over the planarization insulating film 770 and the conductivefilm 772. The insulating film 730 covers part of the conductive film772. Note that the light-emitting element 782 has a top-emissionstructure. Thus, the conductive film 788 has a light-transmittingproperty and transmits light emitted from the EL layer 786. Although thetop-emission structure is described as an example in this embodiment,the structure is not limited thereto. For example, a bottom-emissionstructure in which light is emitted to the conductive film 772 side or adual-emission structure in which light is emitted to both the conductivefilm 772 side and the conductive film 788 side may also be employed.

The coloring film 736 is provided to overlap with the light-emittingelement 782, and the light-blocking film 738 is provided in the leadwiring portion 711 and the source driver circuit portion 704 to overlapwith the insulating film 730. The coloring film 736 and thelight-blocking film 738 are covered with the insulating film 734. Aspace between the light-emitting element 782 and the insulating film 734is filled with a sealing film 732. The structure of the display device700 is not limited to the example in FIG. 29, in which the coloring film736 is provided. For example, a structure without the coloring film 736may also be employed in the case where the EL layer 786 is formed byseparate coloring.

<5-4. Structure Example of Display Device Including Liquid CrystalElement>

The display device 700 illustrated in FIG. 27 includes a liquid crystalelement 775. The liquid crystal element 775 includes a conductive film772, an insulating film 773, a conductive film 774, and a liquid crystallayer 776. In such a structure, the conductive film 774 functions as acommon electrode, and an electric field generated between the conductivefilm 772 and the conductive film 774 through the insulating film 773 cancontrol the alignment state in the liquid crystal layer 776. The displaydevice 700 in FIG. 27 is capable of displaying an image in such a mannerthat transmission or non-transmission is controlled by change in thealignment state of the liquid crystal layer 776 depending on a voltageapplied to the conductive film 772 and the conductive film 774.

The conductive film 772 is electrically connected to the conductive filmfunctioning as the source electrode or the drain electrode of thetransistor 750. The conductive film 772 is formed over the planarizationinsulating film 770 and functions as a pixel electrode, that is, oneelectrode of the display element.

A conductive film that transmits visible light or a conductive film thatreflects visible light can be used as the conductive film 772. Forexample, a material containing an element selected from indium (In),zinc (Zn), and tin (Sn) is preferably used for the conductive film thattransmits visible light. For example, a material containing aluminum orsilver is preferably used for the conductive film that reflects visiblelight. In this embodiment, the conductive film that reflects visiblelight is used for the conductive film 772.

Although FIG. 27 illustrates an example in which the conductive film 772is connected to the conductive film functioning as the drain electrodeof the transistor 750, one embodiment of the present invention is notlimited to this example. For example, the conductive film 772 may beelectrically connected to the conductive film functioning as the drainelectrode of the transistor 750 through a conductive film functioning asa connection electrode.

Although not shown in FIG. 27, an alignment film may be provided incontact with the liquid crystal layer 776. Although not illustrated inFIG. 27, an optical member (optical substrate) and the like such as apolarizing member, a retardation member, or an anti-reflection membermay be provided as appropriate. For example, circular polarization maybe employed by using a polarizing substrate and a retardation substrate.In addition, a backlight, a side light, or the like may be used as alight source.

In the case where a liquid crystal element is used as the displayelement, a thermotropic liquid crystal, a low-molecular liquid crystal,a high-molecular liquid crystal, a polymer dispersed liquid crystal, aferroelectric liquid crystal, an anti-ferroelectric liquid crystal, orthe like can be used. These liquid crystal materials exhibit acholesteric phase, a smectic phase, a cubic phase, a chiral nematicphase, an isotropic phase, or the like depending on conditions.

In the case where a horizontal electric field mode is employed, a liquidcrystal exhibiting a blue phase for which an alignment film isunnecessary may be used. The blue phase is one of liquid crystal phases,which is generated just before a cholesteric phase changes into anisotropic phase when the temperature of a cholesteric liquid crystal isincreased. Since the blue phase appears only in a narrow temperaturerange, a liquid crystal composition in which several weight percent ormore of a chiral material is mixed is used for the liquid crystal layerin order to improve the temperature range. The liquid crystalcomposition containing a liquid crystal exhibiting a blue phase and achiral material has a short response time and optical isotropy, whicheliminates the need for an alignment process. An alignment film does notneed to be provided, and thus, rubbing treatment is not necessary;accordingly, electrostatic discharge damage caused by the rubbingtreatment can be prevented, and defects and damage of a liquid crystaldisplay device in the manufacturing process can be reduced. Moreover,the liquid crystal material that exhibits a blue phase has small viewingangle dependence.

In the case where a liquid crystal element is used as a display element,a twisted nematic (TN) mode, an in-plane switching (IPS) mode, a fringefield switching (FFS) mode, an axially symmetric aligned micro-cell(ASM) mode, an optical compensated birefringence (OCB) mode, aferroelectric liquid crystal (FLC) mode, an anti-ferroelectric liquidcrystal (AFLC) mode, or the like can be used.

Furthermore, a normally black liquid crystal display device such as avertical alignment (VA) mode transmissive liquid crystal display devicemay also be used. There are some examples of a vertical alignment mode;for example, a multi-domain vertical alignment (MVA) mode, a patternedvertical alignment (PVA) mode, and an ASV mode, or the like can beemployed.

At least part of this embodiment can be implemented in combination withany of the other embodiments described in this specification asappropriate.

Embodiment 6

In this embodiment, an example of a display panel which can be used fora display portion or the like in a display device including thesemiconductor device of one embodiment of the present invention isdescribed with reference to FIG. 28 and FIG. 29. The display paneldescribed below as an example includes both a reflective liquid crystalelement and a light-emitting element and can display an image in boththe transmissive mode and the reflective mode.

<6-1. Structure Example of Display Panel>

FIG. 28 is a schematic perspective view illustrating a display panel 600of one embodiment of the present invention. In the display panel 600, asubstrate 651 and a substrate 661 are attached to each other. In FIG.28, the substrate 661 is denoted by a dashed line.

The display panel 600 includes a display portion 662, a circuit 659, awiring 666, and the like. The substrate 651 is provided with the circuit659, the wiring 666, a conductive film 663 which serves as a pixelelectrode, and the like. In FIG. 28, an IC 673 and an FPC 672 aremounted on the substrate 651. Thus, the structure illustrated in FIG. 28can be referred to as a display module including the display panel 600,the FPC 672, and the IC 673.

As the circuit 659, for example, a circuit functioning as a scan linedriver circuit can be used.

The wiring 666 has a function of supplying a signal or electric power tothe display portion or the circuit 659. The signal or electric power isinput to the wiring 666 from the outside through the FPC 672 or from theIC 673.

FIG. 28 shows an example in which the IC 673 is provided on thesubstrate 651 by a chip on glass (COG) method or the like. As the IC673, an IC functioning as a scan line driver circuit, a signal linedriver circuit, or the like can be used. Note that it is possible thatthe IC 673 is not provided when, for example, the display panel 600includes circuits serving as a scan line driver circuit and a signalline driver circuit and when the circuits serving as a scan line drivercircuit and a signal line driver circuit are provided outside and asignal for driving the display panel 600 is input through the FPC 672.Alternatively, the IC 673 may be mounted on the FPC 672 by a chip onfilm (COF) method or the like.

FIG. 28 also shows an enlarged view of part of the display portion 662.The conductive films 663 included in a plurality of display elements arearranged in a matrix in the display portion 662. The conductive film 663has a function of reflecting visible light and serves as a reflectiveelectrode of a liquid crystal element 640 described later.

As illustrated in FIG. 28, the conductive film 663 has an opening. Alight-emitting element 660 is positioned closer to the substrate 651than the conductive film 663 is. Light is emitted from thelight-emitting element 660 to the substrate 661 side through the openingin the conductive film 663.

<6-2. Cross-Sectional Structure Example>

FIG. 29 shows an example of cross sections of part of a region includingthe FPC 672, part of a region including the circuit 659, and part of aregion including the display portion 662 of the display panelillustrated in FIG. 28.

The display panel includes an insulating film 620 between the substrates651 and 661. The display panel also includes the light-emitting element660, a transistor 601, a transistor 605, a transistor 606, a coloringlayer 634, and the like between the substrate 651 and the insulatingfilm 620. Furthermore, the display panel includes the liquid crystalelement 640, a coloring layer 631, and the like between the insulatingfilm 620 and the substrate 661. The substrate 661 and the insulatingfilm 620 are bonded with an adhesive layer 641. The substrate 651 andthe insulating film 620 are bonded with an adhesive layer 642.

The transistor 606 is electrically connected to the liquid crystalelement 640 and the transistor 605 is electrically connected to thelight-emitting element 660. Since the transistors 605 and 606 are formedon a surface of the insulating film 620 which is on the substrate 651side, the transistors 605 and 606 can be formed through the sameprocess.

The substrate 661 is provided with the coloring layer 631, alight-blocking film 632, an insulating film 621, a conductive film 613serving as a common electrode of the liquid crystal element 640, analignment film 633 b, an insulating film 617, and the like. Theinsulating film 617 serves as a spacer for holding a cell gap of theliquid crystal element 640.

Insulating layers such as an insulating film 681, an insulating film682, an insulating film 683, an insulating film 684, and an insulatingfilm 685 are provided on the substrate 651 side of the insulating film620. Part of the insulating film 681 functions as a gate insulatinglayer of each transistor. The insulating films 682, 683, and 684 areprovided to cover each transistor. The insulating film 685 is providedto cover the insulating film 684. The insulating films 684 and 685 eachfunction as a planarization layer. Note that an example where the threeinsulating layers, the insulating films 682, 683, and 684, are providedto cover the transistors and the like is described here; however, oneembodiment of the present invention is not limited to this example, andfour or more insulating layers, a single insulating layer, or twoinsulating layers may be provided. The insulating film 684 functioningas a planarization layer is not necessarily provided when not needed.

The transistors 601, 605, and 606 each include a conductive film 654part of which functions as a gate, a conductive film 652 part of whichfunctions as a source or a drain, and a semiconductor film 653. Here, aplurality of layers obtained by processing the same conductive film areshown with the same hatching pattern.

The liquid crystal element 640 is a reflective liquid crystal element.The liquid crystal element 640 has a stacked structure of a conductivefilm 635, a liquid crystal layer 612, and the conductive film 613. Inaddition, the conductive film 663 which reflects visible light isprovided in contact with the surface of the conductive film 635 thatfaces the substrate 651. The conductive film 663 includes an opening655. The conductive films 635 and 613 contain a material transmittingvisible light. In addition, an alignment film 633 a is provided betweenthe liquid crystal layer 612 and the conductive film 635 and thealignment film 633 b is provided between the liquid crystal layer 612and the conductive film 613. A polarizing plate 656 is provided on anouter surface of the substrate 661.

In the liquid crystal element 640, the conductive film 663 has afunction of reflecting visible light and the conductive film 613 has afunction of transmitting visible light. Light entering from thesubstrate 661 side is polarized by the polarizing plate 656, passesthrough the conductive film 613 and the liquid crystal layer 612, and isreflected by the conductive film 663. Then, the light passes through theliquid crystal layer 612 and the conductive film 613 again and reachesthe polarizing plate 656. In this case, alignment of the liquid crystalis controlled with a voltage that is applied between the conductive film613 and the conductive film 663, and thus optical modulation of lightcan be controlled. That is, the intensity of light emitted through thepolarizing plate 656 can be controlled. Light excluding light in aparticular wavelength region is absorbed by the coloring layer 631, andthus, emitted light is red light, for example.

The light-emitting element 660 is a bottom-emission light-emittingelement. The light-emitting element 660 has a structure in which aconductive film 643, an EL layer 644, and a conductive film 645 b arestacked in this order from the insulating film 620 side. In addition, aconductive film 645 a is provided to cover the conductive film 645 b.The conductive film 645 b contains a material reflecting visible light,and the conductive films 643 and 645 a contain a material transmittingvisible light. Light is emitted from the light-emitting element 660 tothe substrate 661 side through the coloring layer 634, the insulatingfilm 620, the opening 655, the conductive film 613, and the like.

Here, as illustrated in FIG. 29, the conductive film 635 transmittingvisible light is preferably provided for the opening 655. Accordingly,the liquid crystal layer 612 is aligned in a region overlapping with theopening 655 as well as in the other regions, in which case an alignmentdefect of the liquid crystal is prevented from being generated in theboundary portion of these regions and undesired light leakage can besuppressed.

As the polarizing plate 656 provided on an outer surface of thesubstrate 661, a linear polarizing plate or a circularly polarizingplate can be used. An example of a circularly polarizing plate is astack including a linear polarizing plate and a quarter-wave retardationplate. Such a structure can reduce reflection of external light. Thecell gap, alignment, drive voltage, and the like of the liquid crystalelement used as the liquid crystal element 640 are controlled dependingon the kind of the polarizing plate so that desirable contrast isobtained.

In addition, an insulating film 647 is provided on the insulating film646 covering an end portion of the conductive film 643. The insulatingfilm 647 has a function as a spacer for preventing the insulating film620 and the substrate 651 from getting closer more than necessary. Inthe case where the EL layer 644 or the conductive film 645 a is formedusing a blocking mask (metal mask), the insulating film 647 may have afunction of preventing the blocking mask from being in contact with asurface on which the EL layer 644 or the conductive film 645 a isformed. Note that the insulating film 647 is not necessarily providedwhen not needed.

One of a source and a drain of the transistor 605 is electricallyconnected to the conductive film 643 of the light-emitting element 660through a conductive film 648.

One of a source and a drain of the transistor 606 is electricallyconnected to the conductive film 663 through a connection portion 607.The conductive films 663 and 635 are in contact with and electricallyconnected to each other. Here, in the connection portion 607, theconductive layers provided on both surfaces of the insulating film 620are connected to each other through an opening in the insulating film620.

A connection portion 604 is provided in a region where the substrate 651and the substrate 661 do not overlap with each other. The connectionportion 604 is electrically connected to the FPC 672 through aconnection layer 649. The connection portion 604 has a structure similarto that of the connection portion 607. On the top surface of theconnection portion 604, a conductive layer obtained by processing thesame conductive film as the conductive film 635 is exposed. Thus, theconnection portion 604 and the FPC 672 can be electrically connected toeach other through the connection layer 649.

A connection portion 687 is provided in part of a region where theadhesive layer 641 is provided. In the connection portion 687, theconductive layer obtained by processing the same conductive film as theconductive film 635 is electrically connected to part of the conductivefilm 613 with a connector 686. Accordingly, a signal or a potentialinput from the FPC 672 connected to the substrate 651 side can besupplied to the conductive film 613 formed on the substrate 661 sidethrough the connection portion 687.

As the connector 686, a conductive particle can be used, for example. Asthe conductive particle, a particle of an organic resin, silica, or thelike coated with a metal material can be used. It is preferable to usenickel or gold as the metal material because contact resistance can bereduced. It is also preferable to use a particle coated with layers oftwo or more kinds of metal materials, such as a particle coated withnickel and further with gold. As the connector 686, a material capableof elastic deformation or plastic deformation is preferably used. Asillustrated in FIG. 29, the connector 686 which is the conductiveparticle has a shape that is vertically crushed in some cases. With thecrushed shape, the contact area between the connector 686 and aconductive layer electrically connected to the connector 686 can beincreased, thereby reducing contact resistance and suppressing thegeneration of problems such as disconnection.

The connector 686 is preferably provided so as to be covered with theadhesive layer 641. For example, the connectors 686 are dispersed in theadhesive layer 641.

FIG. 29 illustrates an example of the circuit 659 in which thetransistor 601 is provided.

The structure in which the semiconductor film 653 where a channel isformed is provided between two gates is used as an example of thetransistors 601 and 605 in FIG. 29. One gate is formed using theconductive film 654 and the other gate is formed using a conductive film623 overlapping with the semiconductor film 653 with the insulating film682 provided therebetween. Such a structure enables control of thresholdvoltages of a transistor. In that case, the two gates may be connectedto each other and supplied with the same signal to operate thetransistor. Such a transistor can have higher field-effect mobility andthus have higher on-state current than other transistors. Consequently,a circuit capable of high-speed operation can be obtained. Furthermore,the area occupied by a circuit portion can be reduced. The use of thetransistor having high on-state current can reduce signal delay inwirings and can reduce display unevenness even in a display panel inwhich the number of wirings is increased because of increase in size orresolution.

Note that the transistor included in the circuit 659 and the transistorincluded in the display portion 662 may have the same structure. Aplurality of transistors included in the circuit 659 may have the samestructure or different structures. A plurality of transistors includedin the display portion 662 may have the same structure or differentstructures.

A material through which impurities such as water and hydrogen do noteasily diffuse is preferably used for at least one of the insulatingfilms 682 and 683 which cover the transistors. That is, the insulatingfilm 682 or the insulating film 683 can function as a barrier film. Sucha structure can effectively suppress diffusion of the impurities intothe transistors from the outside, and a highly reliable display panelcan be provided.

The insulating film 621 is provided on the substrate 661 side to coverthe coloring layer 631 and the light-blocking film 632. The insulatingfilm 621 may have a function as a planarization layer. The insulatingfilm 621 enables the conductive film 613 to have an almost flat surface,resulting in a uniform alignment state of the liquid crystal layer 612.

An example of the method for manufacturing the display panel 600 isdescribed. For example, the conductive film 635, the conductive film663, and the insulating film 620 are formed in order over a supportsubstrate provided with a separation layer, and the transistor 605, thetransistor 606, the light-emitting element 660, and the like are formed.Then, the substrate 651 and the support substrate are bonded with theadhesive layer 642. After that, separation is performed at the interfacebetween the separation layer and each of the insulating film 620 and theconductive film 635, whereby the support substrate and the separationlayer are removed. Separately, the coloring layer 631, thelight-blocking film 632, the conductive film 613, and the like areformed over the substrate 661 in advance. Then, the liquid crystal isdropped onto the substrate 651 or 661 and the substrates 651 and 661 arebonded with the adhesive layer 641, whereby the display panel 600 can bemanufactured.

A material for the separation layer can be selected such that separationat the interface with the insulating film 620 and the conductive film635 occurs. In particular, it is preferable that a stacked layer of alayer including a high-melting-point metal material, such as tungsten,and a layer including an oxide of the metal material be used as theseparation layer, and a stacked layer of a plurality of layers, such asa silicon nitride layer, a silicon oxynitride layer, and a siliconnitride oxide layer be used as the insulating film 620 over theseparation layer. The use of the high-melting-point metal material forthe separation layer can increase the formation temperature of a layerformed in a later step, which reduces impurity concentration andachieves a highly reliable display panel.

As the conductive film 635, an oxide or a nitride such as a metal oxide,a metal nitride, or an oxide semiconductor whose resistance is reducedis preferably used. In the case of using an oxide semiconductor, amaterial in which at least one of the concentrations of hydrogen, boron,phosphorus, nitrogen, and other impurities and the number of oxygenvacancies is made to be higher than those in a semiconductor layer of atransistor is used for the conductive film 635.

<6-3. Components>

The above components will be described below. Note that descriptions ofstructures having functions similar to those in the above embodimentsare omitted.

[Adhesive Layer]

As the adhesive layer, a variety of curable adhesives such as a reactivecurable adhesive, a thermosetting adhesive, an anaerobic adhesive, and aphotocurable adhesive such as an ultraviolet curable adhesive can beused. Examples of these adhesives include an epoxy resin, an acrylicresin, a silicone resin, a phenol resin, a polyimide resin, an imideresin, a polyvinyl chloride (PVC) resin, a polyvinyl butyral (PVB)resin, and an ethylene vinyl acetate (EVA) resin. In particular, amaterial with low moisture permeability, such as an epoxy resin, ispreferred. Alternatively, a two-component-mixture-type resin may beused. Further alternatively, an adhesive sheet or the like may be used.

Furthermore, the resin may include a drying agent. For example, asubstance that adsorbs moisture by chemical adsorption, such as an oxideof an alkaline earth metal (e.g., calcium oxide or barium oxide), can beused. Alternatively, a substance that adsorbs moisture by physicaladsorption, such as zeolite or silica gel, may be used. The drying agentis preferably included because it can prevent impurities such asmoisture from entering the element, thereby improving the reliability ofthe display panel.

In addition, it is preferable to mix a filler with a high refractiveindex or light-scattering member into the resin, in which case lightextraction efficiency can be enhanced. For example, titanium oxide,barium oxide, zeolite, zirconium, or the like can be used.

[Connection Layer]

As the connection layer, an anisotropic conductive film (ACF), ananisotropic conductive paste (ACP), or the like can be used.

[Coloring Layer]

Examples of a material that can be used for the coloring layers includea metal material, a resin material, and a resin material containing apigment or dye.

[Light-Blocking Layer]

Examples of a material that can be used for the light-blocking layerinclude carbon black, titanium black, a metal, a metal oxide, and acomposite oxide containing a solid solution of a plurality of metaloxides. The light-blocking layer may be a film containing a resinmaterial or a thin film of an inorganic material such as a metal.Stacked films containing the material of the coloring layer can also beused for the light-blocking layer. For example, a stacked-layerstructure of a film containing a material of a coloring layer whichtransmits light of a certain color and a film containing a material of acoloring layer which transmits light of another color can be employed.It is preferable that the coloring layer and the light-blocking layer beformed using the same material because the same manufacturing apparatuscan be used and the process can be simplified.

The above is the description of the components.

<6-4. Manufacturing Method Example>

A manufacturing method example of a display panel using a flexiblesubstrate is described.

Here, layers including a display element, a circuit, a wiring, anelectrode, optical members such as a coloring layer and a light-blockinglayer, an insulating layer, and the like, are collectively referred toas an element layer. The element layer includes, for example, a displayelement, and may additionally include a wiring electrically connected tothe display element or an element such as a transistor used in a pixelor a circuit.

In addition, here, a flexible member which supports the element layer ata stage at which the display element is completed (the manufacturingprocess is finished) is referred to as a substrate. For example, asubstrate includes an extremely thin film with a thickness greater thanor equal to 10 nm and less than or equal to 300 μm and the like.

As a method for forming an element layer over a flexible substrateprovided with an insulating surface, typically, there are two methodsshown below. One of them is to directly form an element layer over thesubstrate. The other method is to form an element layer over a supportsubstrate that is different from the substrate and then to separate theelement layer from the support substrate to be transferred to thesubstrate. Although not described in detail here, in addition to theabove two methods, there is a method in which an element layer is formedover a substrate which does not have flexibility and the substrate isthinned by polishing or the like to have flexibility.

In the case where a material of the substrate can withstand heatingtemperature in a process for forming the element layer, it is preferablethat the element layer be formed directly over the substrate, in whichcase a manufacturing process can be simplified. At this time, theelement layer is preferably formed in a state where the substrate isfixed to the support substrate, in which case transfer thereof in anapparatus and between apparatuses can be easy.

In the case of employing the method in which the element layer is formedover the support substrate and then transferred to the substrate, first,a separation layer and an insulating layer are stacked over the supportsubstrate, and then the element layer is formed over the insulatinglayer. Next, the element layer is separated from the support substrateand then transferred to the substrate. At this time, selected is amaterial with which separation at an interface between the supportsubstrate and the separation layer, at an interface between theseparation layer and the insulating layer, or in the separation layeroccurs. With the method, it is preferable that a material having highheat resistance be used for the support substrate or the separationlayer, in which case the upper limit of the temperature applied when theelement layer is formed can be increased, and an element layer includinga more highly reliable element can be formed.

For example, it is preferable that a stack of a layer containing ahigh-melting-point metal material, such as tungsten, and a layercontaining an oxide of the metal material be used as the separationlayer, and a stack of a plurality of layers, such as a silicon oxidelayer, a silicon nitride layer, a silicon oxynitride layer, and asilicon nitride oxide layer be used as the insulating layer over theseparation layer.

As the method for separating the support substrate from the elementlayer, applying mechanical force, etching the separation layer, andmaking a liquid permeate the separation interface are given as examples.Alternatively, separation may be performed by heating or cooling twolayers of the separation interface by utilizing a difference in thermalexpansion coefficient.

The separation layer is not necessarily provided in the case where theseparation can be performed at an interface between the supportsubstrate and the insulating layer.

For example, glass and an organic resin such as polyimide can be used asthe support substrate and the insulating layer, respectively. In thatcase, a separation trigger may be formed by, for example, locallyheating part of the organic resin with laser light or the like, or byphysically cutting part of or making a hole through the organic resinwith a sharp tool, and separation may be performed at an interfacebetween the glass and the organic resin. As the above-described organicresin, a photosensitive material is preferably used because an openingor the like can be easily formed. The above-described laser lightpreferably has a wavelength region, for example, from visible light toultraviolet light. For example, light having a wavelength of greaterthan or equal to 200 nm and less than or equal to 400 nm, preferablygreater than or equal to 250 nm and less than or equal to 350 nm can beused. In particular, an excimer laser having a wavelength of 308 nm ispreferably used because the productivity is increased. Alternatively, asolid-state UV laser (also referred to as a semiconductor UV laser),such as a UV laser having a wavelength of 355 nm which is the thirdharmonic of an Nd:YAG laser, may be used.

Alternatively, a heat generation layer may be provided between thesupport substrate and the insulating layer formed of an organic resin,and separation may be performed at an interface between the heatgeneration layer and the insulating layer by heating the heat generationlayer. For the heat generation layer, any of a variety of materials suchas a material which generates heat by feeding current, a material whichgenerates heat by absorbing light, and a material which generates heatby applying a magnetic field can be used. For example, for the heatgeneration layer, a material selected from a semiconductor, a metal, andan insulator can be used.

In the above-described methods, the insulating layer formed of anorganic resin can be used as a substrate after the separation.

The above is the description of a manufacturing method of a flexibledisplay panel.

At least part of this embodiment can be implemented in combination withany of the other embodiments and the other examples described in thisspecification as appropriate.

Embodiment 7

In this embodiment, a display device including a semiconductor device ofone embodiment of the present invention is described with reference toFIGS. 30A to 30C.

<7. Circuit Configuration of Display Device>

A display device illustrated in FIG. 30A includes a region includingpixels of display elements (hereinafter referred to as a pixel portion502), a circuit portion that is provided outside the pixel portion 502and includes a circuit for driving the pixels (hereinafter, the circuitportion is referred to as a driver circuit portion 504), circuits havinga function of protecting elements (hereinafter, the circuits arereferred to as protection circuits 506), and a terminal portion 507.Note that the protection circuits 506 are not necessarily provided.

Part or the whole of the driver circuit portion 504 is preferably formedover a substrate over which the pixel portion 502 is formed. Thus, thenumber of components and the number of terminals can be reduced. Whenpart or the whole of the driver circuit portion 504 is not formed overthe substrate over which the pixel portion 502 is formed, the part orthe whole of the driver circuit portion 504 can be mounted by COG ortape automated bonding (TAB).

The pixel portion 502 includes a plurality of circuits for drivingdisplay elements arranged in X (X is a natural number of 2 or more) rowsand Y (Y is a natural number of 2 or more) columns (hereinafter, thecircuits are referred to as pixel circuits 501). The driver circuitportion 504 includes driver circuits such as a circuit for supplying asignal (scan signal) to select a pixel (hereinafter, the circuit isreferred to as a gate driver 504 a) and a circuit for supplying a signal(data signal) to drive a display element in a pixel (hereinafter, thecircuit is referred to as a source driver 504 b).

The gate driver 504 a includes a shift register or the like. The gatedriver 504 a receives a signal for driving the shift register throughthe terminal portion 507 and outputs a signal. For example, the gatedriver 504 a receives a start pulse signal, a clock signal, or the likeand outputs a pulse signal. The gate driver 504 a has a function ofcontrolling the potentials of wirings supplied with scan signals(hereinafter referred to as scan lines GL_1 to GL_X). Note that aplurality of gate drivers 504 a may be provided to control the scanlines GL_1 to GL_X separately. Alternatively, the gate driver 504 a hasa function of supplying an initialization signal. Without being limitedthereto, another signal can be supplied from the gate driver 504 a.

The source driver 504 b includes a shift register or the like. Thesource driver 504 b receives a signal (image signal) from which a datasignal is generated, as well as a signal for driving the shift register,through the terminal portion 507. The source driver 504 b has a functionof generating a data signal to be written to the pixel circuit 501 fromthe image signal. In addition, the source driver 504 b has a function ofcontrolling output of a data signal in response to a pulse signalproduced by input of a start pulse signal, a clock signal, or the like.Furthermore, the source driver 504 b has a function of controlling thepotentials of wirings supplied with data signals (hereinafter referredto as data lines DL_1 to DL_Y). Alternatively, the source driver 504 bhas a function of supplying an initialization signal. Without beinglimited thereto, another signal can be supplied from the source driver504 b.

The source driver 504 b includes a plurality of analog switches, forexample. The source driver 504 b can output, as data signals,time-divided image signals obtained by sequentially turning on theplurality of analog switches. The source driver 504 b may include ashift register or the like.

A pulse signal and a data signal are input to each of the plurality ofpixel circuits 501 through one of the plurality of scan lines GLsupplied with scan signals and one of the plurality of data lines DLsupplied with data signals, respectively. Writing and holding of thedata signal in each of the plurality of pixel circuits 501 arecontrolled by the gate driver 504 a. For example, to the pixel circuit501 in the m-th row and the n-th column (m is a natural number of X orless, and n is a natural number of Y or less), a pulse signal is inputfrom the gate driver 504 a through the scan line GL_m, and a data signalis input from the source driver 504 b through the data line DL_n inaccordance with the potential of the scan line GL_m.

The protection circuit 506 in FIG. 30A is connected to, for example, thescan line GL between the gate driver 504 a and the pixel circuit 501.Alternatively, the protection circuit 506 is connected to the data lineDL between the source driver 504 b and the pixel circuit 501.Alternatively, the protection circuit 506 can be connected to a wiringbetween the gate driver 504 a and the terminal portion 507.Alternatively, the protection circuit 506 can be connected to a wiringbetween the source driver 504 b and the terminal portion 507. Note thatthe terminal portion 507 refers to a portion having terminals forinputting power, control signals, and image signals from externalcircuits to the display device.

The protection circuit 506 electrically connects a wiring connected tothe protection circuit to another wiring when a potential out of acertain range is supplied to the wiring connected to the protectioncircuit.

As illustrated in FIG. 30A, the protection circuits 506 provided for thepixel portion 502 and the driver circuit portion 504 can improve theresistance of the display device to overcurrent generated byelectrostatic discharge (ESD) or the like. Note that the configurationof the protection circuits 506 is not limited thereto; for example, theprotection circuit 506 can be connected to the gate driver 504 a or thesource driver 504 b. Alternatively, the protection circuit 506 can beconnected to the terminal portion 507.

One embodiment of the present invention is not limited to the example inFIG. 30A, in which the driver circuit portion 504 includes the gatedriver 504 a and the source driver 504 b. For example, only the gatedriver 504 a may be formed, and a separately prepared substrate overwhich a source driver circuit is formed (e.g., a driver circuit boardformed using a single crystal semiconductor film or a polycrystallinesemiconductor film) may be mounted.

Each of the plurality of pixel circuits 501 in FIG. 30A can have theconfiguration illustrated in FIG. 30B, for example.

The pixel circuit 501 in FIG. 30B includes a liquid crystal element 570,a transistor 550, and a capacitor 560. As the transistor 550, thetransistor described in the above embodiment can be used.

The potential of one of a pair of electrodes of the liquid crystalelement 570 is set as appropriate in accordance with the specificationsof the pixel circuit 501. The alignment state of the liquid crystalelement 570 depends on data written thereto. A common potential may besupplied to the one of the pair of electrodes of the liquid crystalelement 570 included in each of the plurality of pixel circuits 501. Thepotential supplied to the one of the pair of electrodes of the liquidcrystal element 570 in the pixel circuit 501 may differ between rows.

Examples of a method for driving the display device including the liquidcrystal element 570 include a TN mode, an STN mode, a VA mode, anaxially symmetric aligned micro-cell (ASM) mode, an opticallycompensated birefringence (OCB) mode, a ferroelectric liquid crystal(FLC) mode, an anti-ferroelectric liquid crystal (AFLC) mode, an MVAmode, a patterned vertical alignment (PVA) mode, an IPS mode, an FFSmode, and a transverse bend alignment (TBA) mode. Other examples of themethod for driving the display device include an electrically controlledbirefringence (ECB) mode, a polymer-dispersed liquid crystal (PDLC)mode, a polymer network liquid crystal (PNLC) mode, and a guest-hostmode. Without being limited thereto, various liquid crystal elements anddriving methods can be used.

In the pixel circuit 501 in the m-th row and the n-th column, one of asource electrode and a drain electrode of the transistor 550 iselectrically connected to the data line DL_n, and the other of thesource electrode and the drain electrode of the transistor 550 iselectrically connected to the other of the pair of electrodes of theliquid crystal element 570. A gate electrode of the transistor 550 iselectrically connected to the scan line GL_m. The transistor 550 isconfigured to be turned on or off to control whether a data signal iswritten.

One of a pair of electrodes of the capacitor 560 is electricallyconnected to a wiring through which a potential is supplied (hereinafterreferred to as a potential supply line VL), and the other of the pair ofelectrodes of the capacitor 560 is electrically connected to the otherof the pair of electrodes of the liquid crystal element 570. Thepotential of the potential supply line VL is set as appropriate inaccordance with the specifications of the pixel circuit 501. Thecapacitor 560 functions as a storage capacitor for storing written data.

For example, in the display device including the pixel circuits 501 inFIG. 30B, the gate driver 504 a in FIG. 30A sequentially selects thepixel circuits 501 row by row to turn on the transistors 550, and datasignals are written.

When the transistor 550 is turned off, the pixel circuit 501 to whichthe data has been written is brought into a holding state. Thisoperation is sequentially performed row by row; thus, an image can bedisplayed.

Alternatively, each of the plurality of pixel circuits 501 in FIG. 30Acan have the configuration illustrated in FIG. 30C, for example.

The pixel circuit 501 in FIG. 30C includes transistors 552 and 554, acapacitor 562, and a light-emitting element 572. The transistordescribed in the above embodiment can be used as the transistor 552and/or the transistor 554.

One of a source electrode and a drain electrode of the transistor 552 iselectrically connected to a wiring through which a data signal issupplied (hereinafter referred to as a data line DL_n). A gate electrodeof the transistor 552 is electrically connected to a wiring throughwhich a gate signal is supplied (hereinafter referred to as a scan lineGL_m).

The transistor 552 is configured to be turned on or off to controlwhether a data signal is written.

One of a pair of electrodes of the capacitor 562 is electricallyconnected to a wiring through which a potential is supplied (hereinafterreferred to as a potential supply line VL_a), and the other of the pairof electrodes of the capacitor 562 is electrically connected to theother of the source electrode and the drain electrode of the transistor552.

The capacitor 562 functions as a storage capacitor for storing writtendata.

One of a source electrode and a drain electrode of the transistor 554 iselectrically connected to the potential supply line VL_a. A gateelectrode of the transistor 554 is electrically connected to the otherof the source electrode and the drain electrode of the transistor 552.

One of an anode and a cathode of the light-emitting element 572 iselectrically connected to a potential supply line VL_b, and the other ofthe anode and the cathode of the light-emitting element 572 iselectrically connected to the other of the source electrode and thedrain electrode of the transistor 554.

As the light-emitting element 572, an organic electroluminescent element(also referred to as an organic EL element) can be used, for example.Note that the light-emitting element 572 is not limited thereto and maybe an inorganic EL element including an inorganic material.

A high power supply potential V_(DD) is supplied to one of the potentialsupply line VL_a and the potential supply line VL_b, and a low powersupply potential V_(SS) is supplied to the other of the potential supplyline VL_a and the potential supply line VL_b.

In the display device including the pixel circuits 501 in FIG. 30C, thegate driver 504 a in FIG. 30A sequentially selects the pixel circuits501 row by row to turn on the transistors 552, and data signals arewritten.

When the transistor 552 is turned off, the pixel circuit 501 to whichthe data has been written is brought into a holding state. Furthermore,the amount of current flowing between the source electrode and the drainelectrode of the transistor 554 is controlled in accordance with thepotential of the written data signal. The light-emitting element 572emits light with a luminance corresponding to the amount of flowingcurrent. This operation is sequentially performed row by row; thus, animage can be displayed.

At least part of this embodiment can be implemented in combination withany of the other embodiments described in this specification asappropriate.

Embodiment 8

In this embodiment, a display module and electronic devices, each ofwhich includes a semiconductor device of one embodiment of the presentinvention, are described with reference to FIG. 31, FIGS. 32A to 32E,and FIGS. 33A to 33G.

<8-1. Display Module>

In a display module 7000 illustrated in FIG. 31, a touch panel 7004connected to an FPC 7003, a display panel 7006 connected to an FPC 7005,a backlight 7007, a frame 7009, a printed-circuit board 7010, and abattery 7011 are provided between an upper cover 7001 and a lower cover7002.

The semiconductor device of one embodiment of the present invention canbe used for the display panel 7006, for example.

The shapes and sizes of the upper cover 7001 and the lower cover 7002can be changed as appropriate in accordance with the sizes of the touchpanel 7004 and the display panel 7006.

The touch panel 7004 can be a resistive touch panel or a capacitivetouch panel and overlap with the display panel 7006. Alternatively, acounter substrate (sealing substrate) of the display panel 7006 can havea touch panel function. Alternatively, a photosensor may be provided ineach pixel of the display panel 7006 to form an optical touch panel.

The backlight 7007 includes a light source 7008. One embodiment of thepresent invention is not limited to the structure in FIG. 31, in whichthe light source 7008 is provided over the backlight 7007. For example,a structure in which the light source 7008 is provided at an end portionof the backlight 7007 and a light diffusion plate is further providedmay be employed. Note that the backlight 7007 need not be provided inthe case where a self-luminous light-emitting element such as an organicEL element is used or in the case where a reflective panel or the likeis employed.

The frame 7009 protects the display panel 7006 and functions as anelectromagnetic shield for blocking electromagnetic waves generated bythe operation of the printed-circuit board 7010. The frame 7009 may alsofunction as a radiator plate.

The printed-circuit board 7010 includes a power supply circuit and asignal processing circuit for outputting a video signal and a clocksignal. As a power source for supplying power to the power supplycircuit, an external commercial power source or the separate battery7011 may be used. The battery 7011 can be omitted in the case where acommercial power source is used.

The display module 7000 may be additionally provided with a member suchas a polarizing plate, a retardation plate, or a prism sheet.

<8-2. Electronic Device 1>

Next, FIGS. 32A to 32E illustrate examples of electronic devices.

FIG. 32A is an external view of a camera 8000 to which a finder 8100 isattached.

The camera 8000 includes a housing 8001, a display portion 8002, anoperation button 8003, a shutter button 8004, and the like. Furthermore,an attachable lens 8006 is attached to the camera 8000.

Although the lens 8006 of the camera 8000 here is detachable from thehousing 8001 for replacement, the lens 8006 may be included in thehousing 8001.

Images can be taken with the camera 8000 at the press of the shutterbutton 8004. In addition, images can be taken at the touch of thedisplay portion 8002 that serves as a touch panel.

The housing 8001 of the camera 8000 includes a mount including anelectrode, so that the finder 8100, a stroboscope, or the like can beconnected to the housing 8001.

The finder 8100 includes a housing 8101, a display portion 8102, abutton 8103, and the like.

The housing 8101 includes a mount for engagement with the mount of thecamera 8000 so that the finder 8100 can be connected to the camera 8000.The mount includes an electrode, and an image or the like received fromthe camera 8000 through the electrode can be displayed on the displayportion 8102.

The button 8103 serves as a power button. The display portion 8102 canbe turned on and off with the button 8103.

A display device of one embodiment of the present invention can be usedin the display portion 8002 of the camera 8000 and the display portion8102 of the finder 8100.

Although the camera 8000 and the finder 8100 are separate and detachableelectronic devices in FIG. 32A, the housing 8001 of the camera 8000 mayinclude a finder having a display device.

FIG. 32B is an external view of a head-mounted display 8200.

The head-mounted display 8200 includes a mounting portion 8201, a lens8202, a main body 8203, a display portion 8204, a cable 8205, and thelike. The mounting portion 8201 includes a battery 8206.

Power is supplied from the battery 8206 to the main body 8203 throughthe cable 8205. The main body 8203 includes a wireless receiver or thelike to receive video data, such as image data, and display it on thedisplay portion 8204. The movement of the eyeball and the eyelid of auser is captured by a camera in the main body 8203 and then coordinatesof the points the user looks at are calculated using the captured datato utilize the eye of the user as an input means.

The mounting portion 8201 may include a plurality of electrodes so as tobe in contact with the user. The main body 8203 may be configured tosense current flowing through the electrodes with the movement of theuser's eyeball to recognize the direction of his or her eyes. The mainbody 8203 may be configured to sense current flowing through theelectrodes to monitor the user's pulse. The mounting portion 8201 mayinclude sensors, such as a temperature sensor, a pressure sensor, or anacceleration sensor so that the user's biological information can bedisplayed on the display portion 8204. The main body 8203 may beconfigured to sense the movement of the user's head or the like to movean image displayed on the display portion 8204 in synchronization withthe movement of the user's head or the like.

The display device of one embodiment of the present invention can beused in the display portion 8204.

FIGS. 32C to 32E are external views of a head-mounted display 8300. Thehead-mounted display 8300 includes a housing 8301, a display portion8302, an object for fixing, such as a band, 8304, and a pair of lenses8305.

A user can see display on the display portion 8302 through the lenses8305. It is favorable that the display portion 8302 be curved. When thedisplay portion 8302 is curved, a user can feel high realistic sensationof images. Although the structure described in this embodiment as anexample has one display portion 8302, the number of the display portions8302 provided is not limited to one. For example, two display portions8302 may be provided, in which case one display portion is provided forone corresponding user's eye, so that three-dimensional display usingparallax or the like is possible.

The display device of one embodiment of the present invention can beused in the display portion 8302. The display device including thesemiconductor device of one embodiment of the present invention has anextremely high resolution; thus, even when an image is magnified usingthe lenses 8305 as illustrated in FIG. 32E, the user does not perceivepixels, and thus a more realistic image can be displayed.

<8-3. Electronic Device 2>

Next, FIGS. 33A to 33G illustrate examples of electronic devices thatare different from those illustrated in FIGS. 32A to 32E.

Electronic devices illustrated in FIGS. 33A to 33G include a housing9000, a display portion 9001, a speaker 9003, an operation key 9005(including a power switch or an operation switch), a connection terminal9006, a sensor 9007 (a sensor having a function of measuring force,displacement, position, speed, acceleration, angular velocity,rotational frequency, distance, light, liquid, magnetism, temperature,chemical substance, sound, time, hardness, electric field, current,voltage, electric power, radiation, flow rate, humidity, gradient,oscillation, odor, or infrared ray), a microphone 9008, and the like.

The electronic devices in FIGS. 33A to 33G have a variety of functionssuch as a function of displaying a variety of information (e.g., a stillimage, a moving image, and a text image) on the display portion, a touchpanel function, a function of displaying a calendar, date, time, and thelike, a function of controlling processing with a variety of software(programs), a wireless communication function, a function of beingconnected to a variety of computer networks with a wirelesscommunication function, a function of transmitting and receiving avariety of data with a wireless communication function, and a functionof reading out a program or data stored in a memory medium anddisplaying it on the display portion. Note that functions of theelectronic devices in FIGS. 33A to 33G are not limited thereto, and theelectronic devices can have a variety of functions. Although notillustrated in FIGS. 33A to 33G, the electronic devices may each have aplurality of display portions. Furthermore, the electronic devices mayeach be provided with a camera and the like to have a function of takinga still image, a function of taking a moving image, a function ofstoring the taken image in a memory medium (an external memory medium ora memory medium incorporated in the camera), a function of displayingthe taken image on the display portion, or the like.

The electronic devices in FIGS. 33A to 33G are described in detailbelow.

FIG. 33A is a perspective view illustrating a television device 9100.The television device 9100 can include the display portion 9001 having alarge screen size of, for example, 50 inches or more, or 100 inches ormore.

FIG. 33B is a perspective view of a portable information terminal 9101.The portable information terminal 9101 functions as, for example, one ormore of a telephone set, a notebook, and an information browsing system.Specifically, the portable information terminal 9101 can be used as asmartphone. Note that the portable information terminal 9101 may includea speaker, a connection terminal, a sensor, or the like. The portableinformation terminal 9101 can display text and image information on itsplurality of surfaces. For example, three operation buttons 9050 (alsoreferred to as operation icons or simply as icons) can be displayed onone surface of the display portion 9001. Furthermore, information 9051indicated by dashed rectangles can be displayed on another surface ofthe display portion 9001. Examples of the information 9051 includedisplay indicating reception of an e-mail, a social networking service(SNS) message, or a telephone call, the title and sender of an e-mail oran SNS message, date, time, remaining battery, and reception strength ofan antenna. Alternatively, the operation buttons 9050 or the like may bedisplayed in place of the information 9051.

FIG. 33C is a perspective view of a portable information terminal 9102.The portable information terminal 9102 has a function of displayinginformation on three or more surfaces of the display portion 9001. Here,information 9052, information 9053, and information 9054 are displayedon different surfaces. For example, a user of the portable informationterminal 9102 can see the display (here, the information 9053) on theportable information terminal 9102 put in a breast pocket of his/herclothes. Specifically, a caller's phone number, name, or the like of anincoming call is displayed in a position that can be seen from above theportable information terminal 9102. The user can see the display withouttaking out the portable information terminal 9102 from the pocket anddecide whether to answer the call.

FIG. 33D is a perspective view of a watch-type portable informationterminal 9200. The portable information terminal 9200 is capable ofexecuting a variety of applications such as mobile phone calls,e-mailing, reading and editing texts, music reproduction, Internetcommunication, and a computer game. The display surface of the displayportion 9001 is curved, and display can be performed on the curveddisplay surface. The portable information terminal 9200 can employ nearfield communication conformable to a communication standard. Forexample, hands-free calling can be achieved by mutual communicationbetween the portable information terminal 9200 and a headset capable ofwireless communication. Moreover, the portable information terminal 9200includes the connection terminal 9006 and can perform direct datacommunication with another information terminal via a connector.Charging through the connection terminal 9006 is also possible. Notethat the charging operation may be performed by wireless power feedingwithout using the connection terminal 9006.

FIGS. 33E, 33F, and 33G are perspective views of a foldable portableinformation terminal 9201 that is opened, that is shifted from theopened state to the folded state or from the folded state to the openedstate, and that is folded, respectively. The portable informationterminal 9201 is highly portable when folded. When the portableinformation terminal 9201 is opened, a seamless large display region ishighly browsable. The display portion 9001 of the portable informationterminal 9201 is supported by three housings 9000 joined by hinges 9055.By being folded at the hinges 9055 between the two adjacent housings9000, the portable information terminal 9201 can be reversibly changedin shape from the opened state to the folded state. For example, theportable information terminal 9201 can be bent with a radius ofcurvature greater than or equal to 1 mm and less than or equal to 150mm.

The electronic devices described in this embodiment each include thedisplay portion for displaying some sort of data. Note that thesemiconductor device of one embodiment of the present invention can alsobe used for an electronic device that does not have a display portion.

At least part of this embodiment can be implemented in combination withany of the other embodiments and the other examples described in thisspecification as appropriate.

This application is based on Japanese Patent Application serial no.2016-101581 filed with Japan Patent Office on May 20, 2016, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A method for manufacturing a semiconductor devicecomprising: a first step of forming a first oxide semiconductor filmover a substrate; a second step of heating the first oxide semiconductorfilm; and a third step of forming a second oxide semiconductor film overthe first oxide semiconductor film, wherein the first to third steps areperformed in an atmosphere in which water vapor partial pressure islower than water vapor partial pressure in atmospheric air, and thefirst step, the second step, and the third step are successivelyperformed in this order.
 2. The method for manufacturing a semiconductordevice according to claim 1, wherein the second step is performed withina temperature range of higher than or equal to 100° C. and lower than orequal to 450° C. for a treatment time of longer than or equal to 1minute and shorter than or equal to 60 minutes.
 3. The method formanufacturing a semiconductor device according to claim 1, wherein thefirst oxide semiconductor film and the second oxide semiconductor filmare formed by a sputtering method.
 4. The method for manufacturing asemiconductor device according to claim 1, the first oxide semiconductorfilm is formed under a lower oxygen partial pressure than the secondoxide semiconductor film.
 5. The method for manufacturing asemiconductor device according to claim 1, wherein the first oxidesemiconductor film is formed with an oxygen flow rate ratio of greaterthan or equal to 0% and less than or equal to 50%, and wherein thesecond oxide semiconductor film is formed with an oxygen flow rate ofgreater than or equal to 50% and less than or equal to 100%.
 6. Themethod for manufacturing a semiconductor device according to claim 1,wherein one of the first oxide semiconductor film and the second oxidesemiconductor film is formed at a temperature lower than 100° C.
 7. Themethod for manufacturing a semiconductor device according to claim 1,wherein one of the first oxide semiconductor film and the second oxidesemiconductor film is formed without performing heating intentionally.8. The method for manufacturing a semiconductor device according toclaim 1, wherein the first oxide semiconductor film is formed such thatthe first oxide semiconductor film has lower crystallinity than thesecond oxide semiconductor film.
 9. The method for manufacturing asemiconductor device according to claim 1, wherein the first oxidesemiconductor film is formed such that the first oxide semiconductorfilm comprises a nanocrystal, and wherein the second oxide semiconductorfilm is formed such that the second oxide semiconductor film comprises ac-axis-aligned crystal.
 10. The method for manufacturing a semiconductordevice according to claim 1, wherein one of the first oxidesemiconductor film and the second oxide semiconductor film is formedwith an In-M-Zn oxide target, and wherein M is Ga, Al, Y, or Sn.
 11. Themethod for manufacturing a semiconductor device according to claim 10,wherein an atomic ratio of In to M and Zn is 4:2:4.1 or a neighborhoodof 4:2:4.1.
 12. The method for manufacturing a semiconductor deviceaccording to claim 10, wherein an atomic ratio of In to M and Zn isIn:M:Zn=5:1:7 or a neighborhood of 5:1:7.
 13. The method formanufacturing a semiconductor device according to claim 1, wherein thefirst step, the second step, and the third step are performed in a samedeposition apparatus.
 14. A method for manufacturing a semiconductordevice comprising: a first step of forming a first oxide semiconductorfilm over a substrate; a second step of heating the first oxidesemiconductor film; a third step of forming a second oxide semiconductorfilm over the first oxide semiconductor film; and a fourth step ofheating the substrate, wherein the first to the fourth steps areperformed in an atmosphere in which water vapor partial pressure islower than water vapor partial pressure in atmospheric air, and thefourth step, the first step, the second step, and the third step aresuccessively performed in this order.
 15. The method for manufacturing asemiconductor device, according to claim 14, wherein the second step andthe fourth step are performed in a temperature range of higher than orequal to 100° C. and lower than or equal to 450° C. for a treatment timeof longer than or equal to 1 minute and shorter than or equal to 60minutes.
 16. The method for manufacturing a semiconductor deviceaccording to claim 14, wherein the first oxide semiconductor film andthe second oxide semiconductor film are formed by a sputtering method.17. The method for manufacturing a semiconductor device according toclaim 14, the first oxide semiconductor film is formed under a loweroxygen partial pressure than the second oxide semiconductor film. 18.The method for manufacturing a semiconductor device according to claim14, wherein the first oxide semiconductor film is formed with an oxygenflow rate ratio of greater than or equal to 0% and less than or equal to50%, and wherein the second oxide semiconductor film is formed with anoxygen flow rate of greater than or equal to 50% and less than or equalto 100%.
 19. The method for manufacturing a semiconductor deviceaccording to claim 14, wherein one of the first oxide semiconductor filmand the second oxide semiconductor film is formed at a temperature lowerthan 100° C.
 20. The method for manufacturing a semiconductor deviceaccording to claim 14, wherein one of the first oxide semiconductor filmand the second oxide semiconductor film is formed without performingheating intentionally.
 21. The method for manufacturing a semiconductordevice according to claim 14, wherein the first oxide semiconductor filmis formed such that the first oxide semiconductor film has lowercrystallinity than the second oxide semiconductor film.
 22. The methodfor manufacturing a semiconductor device according to claim 14, whereinthe first oxide semiconductor film is formed such that the first oxidesemiconductor film comprises a nanocrystal, and wherein the second oxidesemiconductor film is formed such that the second oxide semiconductorfilm comprises a c-axis-aligned crystal.
 23. The method formanufacturing a semiconductor device according to claim 14, wherein oneof the first oxide semiconductor film and the second oxide semiconductorfilm is formed with an In-M-Zn oxide target, and wherein M is Ga, Al, Y,or Sn.
 24. The method for manufacturing a semiconductor device accordingto claim 23, wherein an atomic ratio of In to M and Zn is 4:2:4.1 or aneighborhood of 4:2:4.1.
 25. The method for manufacturing asemiconductor device according to claim 23, wherein an atomic ratio ofIn to M and Zn is In:M:Zn=5:1:7 or a neighborhood of 5:1:7.
 26. Themethod for manufacturing a semiconductor device according to claim 14,wherein the first step, the second step, the third step, and the fourthstep are performed in a same deposition apparatus.